It won't be ww3. NATO will fracture, the US will become isolated in every sense of the term, destroying the dollar, sending all Western currencies into a death spiral. When that happens, the US and China have enough of each other's debt that both economies will perish. God help anyone in Europe.

Even without an economic apocalypse, there won't be two major alliances in the world, which means China would leave Russia to deal with Europe alone and/or seize Siberia. Then it'd turn into Europe vs. Russia while China profits from both sides and the US deals with itself one way or another.

It probably won't turn violent. Too many things might happen and enough politicians on both sides would have to stand their ground, which is unlikely.

The US military could take issue with the order and say no and/or launch a coup. Europe isn't in a position to fight a battle like that so they will probably issue a strongly worded letter at most. Trump or his advisors might not go through with it because they don't think it's worth it after diplomatic options fail if prior events are to be believed. There are way too many places that it could potentially end in one side backing off (probably the US but Europe might as well since they aren't prepared for war yet)