Well, I would fully expect an armed response from NATO allies. And for the US to be kicked out of NATO.
Conscription? I predict a flow of young men (and women should the rules be changed) into Canada, Mexico and any other country that will have them. Some will be going to flee the draft. Others will be going to join the militaries of those countries supplying NATO troops, just as some have gone to Ukraine and joined the Forces there. My sons are past draft age, at least they are according to current rules. But they'd both leave the Country if necessary.
Politically, who knows? The Dems would be shouting blue murder about Article 25 and impeachment. But unless they gain a majority they're probably powerless if they 'stay within the law'. Would the GOP say "now he's gone too far!!!!" and take decisive action to get him out of office? I'd love to say 'yes', but I can't.
Would the Military refuse orders to attack Canada or to turn on US citizens? I'd love to say 'no', but once again I can't. He's stacking the Top Brass. And the US military 'rank and file' is chock full of Trump supporters. You'd be amazed at how many. So even if the Brass refused to order troops out, I predict there would be a lot of 'desertion and reformation' as Trump's 'personal army', his own 'Brownshirts'.
As far as domestically, I honestly don't know. The MAGAts that were formerly in our lives were all frothing at the mouth for a chance to grab their guns and 'Defend the Country' from the 'Libtards'. I think there would be a goodly number of Dems who would be prepared to do the same, but we'd be woefully outgunned. Literally. So I don't know if you'd see an actual armed uprising from the left.
I just hope it never comes to pass.