What if it was Canada instead of Ukraine?
What if Canada elected a pro-American government?
What if Russia engineered a coup to install a government friendlier to the Russian cause?

Then, what if the new pro-Russian Canadian government decided to leave the North American Free Trade Agreement (NAFTA) and join the Eurasian Economic Union?
What if they announced they would exit NATO for a mutual defense pact with Russia and China, who would start arming Canada?

What if large pockets of pro-American Canadians along the border took up arms against Ottawa, igniting a civil war?

What would the U.S. do?
Declare that Russian aggression cannot be allowed to stand?
Mount a military operation to liberate Ottawa and restore democracy and liberty in the Great White North?

By Edward Thomas using Canva

It stands to reason that a country cannot allow another country to create a hostile state on its border.