1.) Do nothing. Poland, and the parts of Germany that used to be East Germany, might not like the long term consequences of this plan.
2.) Start a nuclear war, killing hundreds of millions of people. A few disadvantages to this plan as well.
3.) Economic sanctions. You can certainly say this will kill people by way of depressing GDP growth. Will it kill fewer people than a full nuclear exchange between the US and Russia?
|US invasions| >> |Russia invasions|
But maybe it is because of the difference in GDP: |US invasions| / GDP ~= |Russia invasions| / GDP4) The democratic way: ask people in this "another" country and if the opinions split then restructure it: either convert it peacefully into two countries (like Czechoslovakia) or confederation/federation.
Both of these are the best exports of American influence and require the US to be at the center of all things financial.
That was all nice and fine. Unfortunately it's no more sustainable with a president, who can kill a country's economy with an early morning twitter tantrum.