In fact, alliances can encourage confrontation by emboldening weaker, sometimes irresponsible partners. In 2008 Georgia appeared to believe that Washington would back it against Russia. Offering military support to Ukraine could have a similar effect. That would prove especially dangerous in a region that will always remain more important to Russia than to America. In the end, the U.S. could find itself practically alone fighting a war with a nuclear power over minimal geopolitical stakes. Washington should bar further NATO expansion, whether de jure or de facto. Over the longer term America should turn responsibility for Europe’s defense back to Europe.