The US could also retaliate economically. It is the largest economy in the world and possesses the most attractive consumer market. Germany threatens to "punish" the US -> No more importing BMWs. Unfortunately Europes economy has lagged for so many years that it is unrealistic to threaten economic sanctions
The US can go one step further very easily, by doing nothing. All it has to do is look the other way for five minutes and Russia will begin reassembling its empire in eastern Europe. It would immediately begin taking territory back. Russia would rip Europe apart, trivially, because of how weak Western Europe is militarily today. It's entirely unable to defend itself, outside of France and the UK having nukes (which they're not going to use to stop Russia in eastern Europe).
The US could also let Russia begin messing with Canada's northern territory. Canada is entirely unable to defend itself, with their tiny 1% military budget. There's nothing Canada could do to stop Russia.
The US could let China take its historic anger out on Japan; they would end up conquering Japan one way or another. The US could pull troops out of South Korea, the North would immediately conquer / obliterate the South. China would immediately take Taiwan, with hardly a fight at all.
China would then move to become far more aggressive toward Australia, viewing it as a potential resource colony. Australia's only defense option would be to develop nukes as fast as possible.
If the US stood down in Latin America as the overwatch in the hemisphere, those countries would begin killing each other for territorial conquest. Venezuela is the obvious prize there to be invaded by Brazil or Colombia. Lots of smaller Latin American nations would fall.
The world as we know it would collapse rapidly if the US just looked the other way and stopped restraining the type of conquest behavior that pervades all of human history. That would of course be to its own detriment if it went too far, as authoritarian China would likely then rule the planet, perhaps for a very long time.
