The United States entered the Great War with its eyes wide open. The mechanical slaughter in Europe had already left millions dead. In the trenches, men had to contend with lice, rats, sickness, mud, extreme temperatures, human waste, rotting corpses, and boredom as well as the threats of poison gas, explosive shells, and being buried alive. In 1914, Europe went to war with only the dimmest awareness of the horrors to come. Yet Congress voted overwhelmingly for a declaration of war in the absence of any direct threat to U.S. territory and despite the country’s long tradition of distancing itself from European wars. What could explain both the American government’s decision and the broad and deep popular support for the war?

Today, even a well-rounded college graduate is unlikely to know more about American intervention than the fact that it had something to do with German submarines. Yet why did the United States send two million men to fight in France and Belgium after the Germans sank a handful of merchant vessels? The answer is that Americans across the political spectrum believed they were fighting to defend their inalienable rights, which included the freedom of the seas. If the United States let the German empire trample on its rights, this weakness would invite other challenges. There seemed to be no option but war.

By the 1930s, however, a new consensus portrayed American intervention as a tragic and wasteful misadventure. The Allies’ hollow victory did not bring peace, but only pervasive fears of a more destructive war to come. In today’s terminology, Americans retroactively redefined intervention in the Great War as a choice, not a necessity. The simplistic distinction between wars of choice and wars of necessity, however, only clouds our understanding of the past and its lessons for today. If one revisits 1917 without these conceptual blinders on, what begins to emerge is a deeper understanding of what Americans, in any time and place, believe is worth fighting for.

Wars of Necessity, Wars of Choice

In theory, wars of necessity have a justification so compelling that there is effectively no choice but to fight. World War II provides the paradigmatic example. For the United States, however, wars of necessity are the exception, not the rule. The label does not account for the War of 1812, the Mexican-American War, the Spanish-American War, the Korean War, Vietnam, the invasions of Panama and Grenada, or George W. Bush’s war in Iraq. It certainly does not account for humanitarian interventions. Even George H. W. Bush’s war in Iraq does not measure up. General Colin Powell, then serving as chairman of the Joint Chiefs of Staff, opposed the decision to fight. So did 47 senators, whereas only 23 voted against the war to oust Saddam Hussein.

In early 2003, the distinction between wars of choice and necessity earned its place in our political lexicon because it seemed to capture the difference between the invasion of Afghanistan and the impending war with Iraq. In 2009, President Obama justified the dispatch of additional troops to Afghanistan by insisting it remained a war of necessity, yet within months he had shifted to emphasizing his interest in ending the war. Hindsight transformed a necessity into a choice, as it had after World War I.

To dispel this confusion, the place to begin is with the old saw that war should always be the option of last resort. The question is, The last resort before what? The answer depends greatly on both Americans’ understanding of their role in the world and their assessment of other countries’ capabilities and intentions. This is the real terrain on which a debate about war and peace should take place.

The Meaning of American Neutrality, 1914-1916

In response to the outbreak of war in Europe, President Woodrow Wilson implored Americans to be “impartial in thought as well as in action.” This high-minded sentiment was impractical not just in terms of psychology, but, more important, in terms of geography and technology. According to diplomatic convention, neutral powers had a right to commerce with all belligerents, including the right to sell munitions and war materiel. In practice, Britain’s naval supremacy ensured that only the Allies would have access to the American arsenal. Germany soon discovered that submarines were the only means of disrupting the commerce that sustained the Allies’ war effort.

Diplomatic conventions conceived in the age of sail and steam were not readily adjustable to the advent of submarine warfare. In theory, the German Navy had the right to sink any vessel carrying war materiel to Allied ports after conducting an inspection of its cargo and after ensuring the safety of the crew. Observing such conventions entailed tremendous risks for German submarines, since they moved slowly and had thin hulls. Their main advantage was surprise, yet they had to surface in order to inspect Allied cargo vessels. At that point, a cargo ship could ram the U-boat or, if it were armed, direct its fire at a stationary target.

The tension inherent in this mismatch between technology and tradition ensured that submarine warfare would become a flashpoint in German-American relations during the first years of the war. While many U-boats scrupulously followed the rules, some did not. On May 7, 1915, only three months after the onset of submarine warfare, U-20 sank the Lusitania, a British passenger liner, without warning. The Lusitania sank in only 18 minutes, resulting in the deaths of almost 1,200 passengers, among them 128 Americans. Whole families perished. Paul and Gladys Crompton died along with all six of their children, one still an infant.

There was widespread outrage in the United States, yet few calls for war. Publicly, the Germans refused to concede either the immorality or the illegality of the sinking, yet they maintained a commitment to respect the rights of neutrals. Privately, the kaiser expressed deep regret, later telling the American ambassador in Berlin he would have forbidden the sinking if he could have, since “no gentleman would kill so many women and children.” Accordingly, the kaiser issued secret orders to the navy forbidding future attacks on large passenger ships.

For more than a year, heated diplomatic exchanges followed every significant infringement of the rules by a German U-boat. Yet during the entire period of neutrality, only three Americans sailing on American ships lost their lives as a result of a U-boat attack. Another 62 perished while aboard British or other belligerent vessels. According to John Milton Cooper, the most prominent historian of Wilson’s presidency, “the threat of war was in remission” during the latter half of 1916.

The Rupture of U.S.-German Relations

American voters reelected Woodrow Wilson by a three-point margin in November 1916, although a shift of only 4,000 votes in California would have given a majority in the Electoral College to Republican candidate Charles Evans Hughes. Wilson’s advocates relentlessly reminded voters, “He kept us out of war.” Yet less than a month into his second term, Wilson asked Congress for a declaration of war on Germany.

On January 31, 1917, the German ambassador in Washington, Count Johann von Bernstorff, informed Secretary of State Robert Lansing that on the following day the German Navy would initiate a campaign of unrestricted submarine warfare. In other words, Germany would embark on the wholesale violation of laws of war it had repeatedly acknowledged as binding over the past two years. American ships would be sunk without warning. The announcement came as a shock to Washington.

Although wartime propaganda would later portray Germany as the land of bestial Huns, Americans mostly considered the Reich to be a civilized nation, despite its frequently unbecoming conduct. There was little sense that Wilhelmine Germany was a criminal state run by brutal extremists, like the future Third Reich.

In response to the German announcement, President Wilson embraced a policy of incredulity. He told a joint session of Congress on February 3 that while he had no choice but to sever diplomatic ties,

I refuse to believe that it is the intention of the German authorities to do in fact what they have warned us they will [do]. I cannot bring myself to believe that they will .  .  . destroy American ships and take the lives of American citizens in the willful prosecution of the ruthless naval program they have announced their intention to adopt.

While enraged by the German announcement, both congressmen and newspaper editors overwhelmingly supported Wilson’s policy.

Armed Neutrality

At first, German behavior seemed to vindicate Wilson’s optimism. The same day as Wilson’s address to Congress, the submarine U-53 torpedoed the American freighter Housatonic. Before sinking the Housatonic, the U-53 came to the surface, where its captain emerged to explain in fluent English, “You are carrying foodstuffs to an enemy of my country, and though I am sorry, it is my duty to sink you.” The Germans deposited the crew of the Housatonic safely on the English coast. Nine days later, the U-35 sank another American vessel in a similarly courteous manner. Wilson did not protest either incident, though fear of attack had paralyzed the American merchant fleet.

Wilson held fast to his policy of inaction until the revelation of the Zimmermann Telegram. British intelligence had intercepted a communiqué, which it shared with Washington, in which the Germans offered to reward Mexico with Texas, Arizona, and New Mexico if it would join the German side in the event of war. Wilson knew the proposal was utterly implausible. Nonetheless, it damaged his hope that the Germans shared his determination to preserve the peace.

On February 26, the day after learning of the telegram—but before informing the press of its existence—Wilson returned to Congress to assert that “the situation we find ourselves in with regard to the actual conduct of the German submarine warfare .  .  . is substantially the same that it was when I addressed you on the third of February.” However, change was necessary because American ships were afraid to leave port. Wilson requested support for a policy of “armed neutrality” that would entail the provision of defensive armaments to American ships. The House quickly passed an armed neutrality bill, 403 to 13. There was overwhelming support in the Senate, yet a filibuster by four antiwar progressives killed the proposal. While requesting congressional support, Wilson made clear his belief that his authority as commander in chief enabled him to act without legislative approval, which he soon did.

Wilson exercised an agonizing degree of patience. Germany did not retreat an inch from its official policy of unrestricted submarine warfare, yet neither did its behavior dash Wilson’s hope of a German desire for peace, which in fact existed only in his imagination. Two months earlier, at an Imperial Conference at the castle of Pless, the kaiser made an irrevocable decision to suffocate the British Isles. He and his advisers believed this would knock Britain out of the war and force the remaining Allies to sue for peace. The German leadership fully expected the United States to wage war rather than surrender the commercial rights it had asserted for so long. The Germans knew that Americans would consider submarine warfare to be an intolerable insult. But they calculated that the United States could not mobilize quickly enough to prevent a German victory.

There was nothing disingenuous about the Reich’s announcement of its decision in February. Its leaders did not intend to play games with Wilson, or to take advantage of his desperate desire for peace. Forty-five days passed between the German announcement and the first incontrovertible act of war because the United States was an afterthought. In the words of German naval minister Eduard von Capelle, “From a military point of view America is as nothing.”

No Choice But War

On March 18, Americans learned of the sinking of the Vigilancia by a German submarine. The torpedo struck without warning. “The steamer sank in seven minutes; its captain never saw the attacking U-boat. [The Vigilancia] flew an American flag. Its name and home city were painted on port and starboard bows in letters five feet high and could be read at a distance of three miles,” writes diplomatic historian Justus Doenecke. Fifteen crewmen drowned.

This was the incontrovertible act of war that Wilson had feared. This was the indication that all of his yearning for peace was worthless. Yet Wilson maintained a remarkable silence. Sensing that war was imminent, socialist, pacifist, and German-American organizations launched a final campaign of antiwar protests. With the tension almost unbearable, the American public had to wait 15 days before hearing from the commander in chief.

On March 20, Wilson called his cabinet into session for two-and-a-half hours. To a man, they called for war. At the conclusion of the meeting, Wilson said only, “I think there is no doubt as to what your advice is.” Three days later, Wilson announced he would address Congress on April 2.

The implementation of armed neutrality had only just begun, yet Wilson and his advisers judged it to be unworkable. Neutrality ruled out any offensive actions. A merchant vessel’s only option was to spot a submerged U-boat and pull the trigger first. On April 1, the U-46 sank the Aztec, an armed American transport; 28 men died, including a naval gunner.

The next day, Wilson delivered his war message to Congress. Today, one phrase in particular from that address remains embedded in public memory: “The world must be made safe for democracy.” It has become shorthand for the mistaken belief that Wilson’s war was a global crusade for freedom. It was nothing of the kind. In the passages dedicated to justifying a declaration of war, Wilson focused overwhelmingly on the perfidy of unrestricted submarine warfare and the insufficiency of armed neutrality. The Senate voted for war 82-6, the House 373-50.

American forces would deliver terrific blows to the Reich in their offensives on the Marne, at St. Mihiel, and on the Meuse-Argonne front. Tens of thousands would remain behind, lying to this day under rows of small white crosses.

The Illusions of Hindsight

In the 1920s and ’30s, progressives recast the Great War not simply as a war of choice, but as a war of greed and malice. While the settlement at Versailles disintegrated and Americans rejected responsibility for defending it, progressives revived old conspiracy theories about the role of arms merchants in provoking the 1917 intervention. Before voting against the war, Sen. George Norris (R-Neb.) declared, “We are going into war upon the command of gold. We are going to run the risk of sacrificing millions of our countrymen’s lives in order that other countrymen may coin their lifeblood into money. .  .  . I feel that we are about to put the dollar sign on the American flag.”

Progressives imposed this interpretation so effectively that even Franklin Roosevelt had to pay it lip service while swimming against the isolationist tide. In the “I Hate War” address of 1936—commemorated today as part of the FDR Memorial in Washington D.C.—Roosevelt warned, “If war should break out again in another continent, let us not blink the fact that we would find in this country thousands of Americans who, seeking immediate riches—fools’ gold—would attempt to break down or evade our neutrality.”

The second great war against Germany reversed the widespread assumption that the first had been unnecessary. Americans regretted their complacent dismissal of Wilson’s prophetic warnings about the need for collective security.

Since World War II overshadowed its predecessor so decisively, further debate about Wilson’s foreign policy has not resonated beyond the walls of the academy. For most Americans, World War I has become little more than the first good war against very bad Germans.

Nonetheless, persistent debates among scholars demonstrate the instability of any judgment about whether a given conflict is a war of choice or a war of necessity. Since 1945, “realist” scholars of international relations have advocated the unusual hypothesis that World War I was a sort of accidental war of necessity. Emblematic of the realist perspective is Henry Kissinger’s withering criticism of Wilson as a naïve crusader to whom “national interests were irrelevant” and for whom “the war had a moral foundation, whose primary objective was a new and more just international order.” In contrast, a shrewd president would have recognized the imperative to enter the war in order to prevent Germany from dominating Europe and someday threatening the United States.

Strangely, Kissinger asserts, “Wilson did not justify America’s entry into the war on the grounds of specific grievances” against Germany. Of course, that is exactly what Wilson did. Few scholars, however, have found Wilson’s logic to be compelling on strategic grounds. According to Doenecke, “The president realized American security was not in jeopardy.” But its honor and prestige were threatened.

Wilson’s biographers are sympathetic to his decision, yet portray it as a moral tragedy rather than a strategic masterstroke. John Milton Cooper emphasizes how the final words of Wilson’s war message are borrowed from Martin Luther’s response to accusations of heresy: “God helping me, I can do no other.” In the end, Germany’s criminal behavior overcame Wilson’s revulsion at the savagery of war.

Beyond Choice and Necessity

The esoteric history of neutrality laws and submarine warfare should not obscure the basic truth at the heart of Wilson’s justification for war. World order depends on the existence of rules, even if their application is difficult and there is no impartial transnational authority to enforce them. When a nation of Germany’s stature flagrantly violated those rules, it posed a potential threat to every state that relied on the existing order for its security.

Some, like Holland and Norway, remained neutral because of their relative weakness. If the United States, circa 1917, had considered itself to be more like the Dutch and Norwegians than the British, French, and Germans, it could have accepted German impositions and resigned itself to whatever outcome the war generated. Instead, Americans took it for granted that the United States ought to employ its power to defend and shape the order on which its security depended.

At an earlier point in time, Americans would not have understood their role this way. In the interwar years, Americans would return to a more restricted sense of their responsibilities and interests.

Today, Americans are struggling to understand their role in the world. After seven decades as a superpower, there is still broad acceptance of the idea that the United States ought to be a world leader or even the “indispensable nation” that protects the prevailing liberal order. At the same time, there is growing fear of the costs of leadership, especially the possibility of war with dangerous adversaries such as Russia, China, or Iran.

As in Wilson’s time, Americans want the benefits of order while remaining uneasy about the costs. This does not mean that the United States must respond with force every time that order is threatened. It may reconcile itself to Russia’s flagrant violation of Ukrainian sovereignty. It may reconcile itself to Beijing’s intimidation in the South China Sea, Iran’s pursuit of nuclear weapons, and the rise of a terrorist protostate within Syria and Iraq. What cannot be known is whether and when a crumbling order will bring the threat of violence directly to American shores.

The choice confronting Americans is whether to remain the kind of country that will act before its back is against the wall, or whether it will accept whatever kind of security environment emerges in the absence of American leadership. The advantage of being proactive is that the United States can respond to threats before they achieve maximum lethality. The disadvantage is that Americans will never know, even in hindsight, whether a war was truly necessary. What would have been the impact of a German victory in the Great War, a Communist occupation of South Korea, or Saddam’s annexation of Kuwait? No one will ever know for sure.

Describing wars in terms of choice or necessity blinds America to the uncertainty inherent in the pursuit of national security. If the United States remains committed to defending the liberal order it created, the most pressing question will always be how to choose wisely when considering the use of force.

David Adesnik is a visiting fellow at AEI’s Marilyn Ware Center for Security Studies.

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