The failures of American will exposed by Russia’s invasion of Ukraine are numerous and mounting. Coming on top of the tepid response to China’s declaration of an air defense identification zone over Japanese waters and the withdrawals from Iraq, Afghanistan, and the “red line” in Syria, they have revealed Barack Obama as a man who not only “leads from behind” but marches to the rear.

In some ways, however, critiques of Obama’s weakness may be as revealing of America’s new strategic timidity as the president’s policy. Take, for example, Charles Krauthammer’s recent “How to Stop Putin” essay in the Washington Post. Krauthammer neatly eviscerates Obama’s policy. He goes on to describe what an effective response would consist of: reassuring NATO, deterring further Russian adventurism in Ukraine, and reversing the annexation of Crimea. But having imagined what a victory would look like, the clear-eyed columnist tries to reassure us: “This is no land-war strategy,” he writes. “This is the ‘trip-wire’ strategy successful for half a century in Germany and Korea.”

But what was that trip-wire? It was a thin skein of U.S. Army and other land-force units stretched along the “inner-German border” and the 38th parallel on the Korean peninsula. In Korea, some of that trip-wire is still there, staring through its binoculars at the enemy. A good deal of what deterred the Soviets—and still deters even the nutty Norks—is the knowledge that, should they test the trip-wire, they run a serious risk of engaging with American ground troops. Ground troops who wear boots, patrolling the front lines of what in a less ironic time was called the frontier of freedom.

In the post-Iraq era, even conservatives and Republicans have internalized the no-boots-on-the-ground catechism. Sen. John McCain argued both for intervention in Syria and that “the worst thing the United States could do right now is put boots on the ground in Syria.” That was a year ago; since then the Syrian opposition has consistently lost ground to the forces of the Assad regime while al Qaeda affiliates, who love nothing better than controlling territory, likewise prosper. Conservatives should not will the ends without willing the means, or divert their eyes from military realities.

It is no surprise that the focus of the Obama defense cuts has been on land forces, and particularly the U.S. Army. Putting boots on the ground is not merely a statement of American political commitment, but often the only means to be militarily and strategically decisive. Conversely, if your goal is to prevent the United States from exercising a decisive influence, then you take those tools away. But if conservatives agree that the United States is out of the land war business, then they will have to concede that there’s a method to Obama’s madness.

Krauthammer begins his Crimea column by mocking the president of the Los Angeles World Affairs Council for snidely asking, “What are [we] going to do, send the 101st Airborne into Crimea?” And indeed, putting the Screaming Eagles directly into Crimea would be a tactical blunder. But putting one brigade astride each of the two main roads—and there are only two—that connect Crimea to the Ukrainian mainland would not be militarily silly, if backed by U.S. aircraft and partnered with NATO and Ukrainian units. In 1990, it was the deployment of the 82nd Airborne to Saudi Arabia—the troops called themselves “speed bumps”—that helped Saddam Hussein decide to stop at the Kuwait border.

Russia’s tactics in Crimea—using small, elite, stealthy but lightly armed units with limited mobility and with the advantage of support from Russian bases in Crimea—cannot so easily be replicated even in eastern Ukraine. And last week’s events, such as the move on the Ukrainian naval headquarters in Sevastopol, testify that the Russians still have substantial work to do to solidify their grip on Crimea. These tactics betray an essential Russian weakness: Their larger conventional forces have been left to rot since the Soviet era. Russian land-force performance in the 2008 Georgia war was poor, and even Georgia’s air defenses proved strong enough to deter the large air assault the Russians considered. Putin is a bold man, but one playing a weak and brittle military hand.

It would be politically courageous to call his bluff and find out what cards Putin really holds, but no American—no Western—politician seems willing to cover that bet with boots on the ground. That is a crippling weakness as, after a generational vacation from history, post-Cold-War strategic competition begins in earnest, not just in Europe, but across the Middle East and throughout East Asia. Power abhors a vacuum, except when—as we see with Vladimir Putin, Ali Khamenei, and Xi Jinping—power covets it.

Ukraine is still, for the present, a no-man’s-land, neither West nor East. But Ukraine is hardly the only no-man’s-land. The entire Middle East is fast becoming an especially gruesome one. The South China Sea is likewise up for grabs. Absent a constant and powerful military presence by the forces of an American-led coalition, our adversaries are laying claim to these no-man’s-lands.

The United States still possesses uniquely powerful air and naval forces that, in the South China Sea, Persian Gulf, or Arabian Sea, can be sufficient to patrol the perimeter and deter conflict. Yet this capability cannot defeat geography. Preserving the peace on the Eurasian landmass demands land forces. These need not be very large—they can indeed be a trip-wire, if backed up by airpower and reserves—but they have to be there.

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