John J. Mearsheimer writes: The Ukraine crisis is almost a year old and Russia is winning. The separatists in eastern Ukraine are gaining ground and Russia’s president, Vladimir V. Putin, shows no signs of backing down in the face of Western economic sanctions.

Unsurprisingly, a growing chorus of voices in the United States is calling for arming Ukraine. A recent report from three leading American think tanks endorses sending Kiev advanced weaponry, and the White House’s nominee for secretary of defense, Ashton B. Carter, said last week to the Senate armed services committee, “I very much incline in that direction.”

They are wrong. Going down that road would be a huge mistake for the United States, NATO and Ukraine itself. Sending weapons to Ukraine will not rescue its army and will instead lead to an escalation in the fighting. Such a step is especially dangerous because Russia has thousands of nuclear weapons and is seeking to defend a vital strategic interest.

There is no question that Ukraine’s military is badly outgunned by the separatists, who have Russian troops and weapons on their side. Because the balance of power decisively favors Moscow, Washington would have to send large amounts of equipment for Ukraine’s army to have a fighting chance.

But the conflict will not end there. Russia would counter-escalate, taking away any temporary benefit Kiev might get from American arms. The authors of the think tank study concede this, noting that “even with enormous support from the West, the Ukrainian Army will not be able to defeat a determined attack by the Russian military.” In short, the United States cannot win an arms race with Russia over Ukraine and thereby ensure Russia’s defeat on the battlefield. [Continue reading…]