President Joe Biden is not planning to answer a further Russian invasion of Ukraine by sending combat troops. But he could pursue a range of less dramatic yet still risky military options, including supporting a post-invasion Ukrainian resistance.

The rationale for not directly joining a Russia-Ukraine war is simple. The United States has no treaty obligation to Ukraine, and war with Russia would be an enormous gamble, given its potential for expanding in Europe, destabilizing the region, and escalating to the frightening point of risking a nuclear exchange.

Doing too little has its risks, too. It might suggest an acquiescence to future Russian moves against other countries in eastern Europe, such as the Baltic states of Estonia, Latvia and Lithuania, although as NATO members those three have security assurances from the United States and the rest of the alliance.