What would be the purpose of US support in Europe if not defending against Russia?

The US presence in Europe exists to facilitate US power projection, if we are not stopping Russian advancement then there is no reason for host countries to allow US bases in their territory.

The population of the US has no appetite for invading Canada, many of us have close friends and family there, we consider Canada a friend and ally and Trumps hissy fit doesn’t change that.

If the US invades Canada there will be a lot of Americans aiding resistance.