Before the invasion, Mr. Putin appears to have become overconfident in his own judgment and his armed forces after successfully seizing Crimea in 2014 and helping Syria’s Bashar al-Assad stay in power since 2015—much as the U.S. did after the Gulf War in 1991, the Kosovo effort in 1999 and the toppling of the Taliban in 2001. Combine that overconfidence with wounded pride about Russia’s place in the world and anger at his adversaries, and you have a psychology capable of making a big miscalculation—without needing insanity to explain the misjudgment.

What about the nuclear threats? There are precedents for Mr. Putin’s putting his forces on alert early in the invasion: The U.S. did the same during the 1973 Middle East war to discourage Soviet intervention. The Russian leader’s pre-invasion vow to meet resistance with “consequences that you have never experienced in your history” was most likely an attempt at deterrence through intimidation.

The implications of all this are twofold. First, Mr. Putin has probably begun to realize he made a mistake. He may look for a way to mitigate it. The West should consider whether there is a way to ensure Ukraine’s long-term security without North Atlantic Treaty Organization membership, in exchange for Russian withdrawal and commitment to a verifiable arrangement. Second, if Mr. Putin is rational, NATO can deter him from attacks against Poland and the Baltics by moving more troops there.