Should the United States arm Ukraine for its fight against Russian President Vladimir Putin? Before you say "Duh," consider this: Arms shipments alone are almost never enough to enable a smaller, weaker actor to defeat a big-time power. If the U.S. commits itself to sending arms to Ukraine, it's signing up for more than military aid.

When Ukraine needs more help, America's credibility will be on the line — and pressure will be great to escalate even to the point of air support.

On the surface, this analysis might seem extreme. Right now, nobody on the American political spectrum, not even John McCain, thinks the U.S. should be prepared to fight an air war with Russia. That this idea is almost unthinkable, however, is a good reason to play out the various scenarios associated with sending arms to Ukraine.