I'm not saying World War III, but I imagine at a minimum we will have boots on the grounds before too long to "keep the peace."
There is zero chance we will have troops in the Ukraine, Zero.. In an election year, no less, nobody is gonna want to propose sending our guys to a rats nest like they have there.
If the Russians did shoot this plane down the only thing it might do is have them withdraw support, though I really doubt that happens. The best way to hit the Russians, is financially , they are now part of the world economy and have enjoyed much of what free enterprise has to offer.