The responses may involve a combination of diplomatic isolation and increasingly devastating economic sanctions as well as military aid or more covert intelligence assistance to Ukraine. The precise details of various actions could be debated. But the general calculus from an American perspective should be that we should take actions that will impose a greater cost on Putin than they would on us, and we should avoid actions that would prove more costly to the U.S. relative to the benefit.

Ultimately, what happens will depend on how hard the Ukrainian people are willing to fight and how long Putin can maintain support at home for the invasion. While Russia has a more powerful military, Ukraine has a large population, a challenging climate, and imposing urban areas. The logistics of sustaining an invasion across such a large territory will be enormous. If Ukrainians are willing to absorb casualties and keep an insurgency going, the invasion could become tremendously costly for Russia. While reports from the battlefield should always be viewed with caution, early signs point to Ukrainians putting up a fight. Ukraine has also announced the distribution of 10,000 rifles to civilians to defend Kyiv.

While, as an autocrat, Putin is not restrained by public will, he is also not impervious to the effects of waning public support or low morale.