The fact is Obama signed the agreement July 2016.
That gives the UN and UN countries legal right to invade with force over civilians to "protect civilians" in times of disaster or civil war, etc.
What is the USA going to do nuke all the countries of the UN that say they are helping civilians in the USA? I don't think so especially not if their approved person signed the document that makes it legal.
In my opinion, the legalities of international agreements aren't worth the paper they are written on. They are just general guidelines for various operational understandings between nations, and only when such operations benefit both nations involved or one has no other options.
For reference, I would submit exhibit 1) The number of times the USA has gone against its agreements with other countries such as Iran and the environmental agreements.
Here in the USA, we have a culture where rules only apply to others, not the people who make the rules.. and this same culture is in play when we make international agreements.
Everyone's opinion on likelihood is always up for debate.
The point here is that legal doors are already in place to allow a possible legal invasion based on humanitarian reasons from civil unrest caused by who knows what events but could include EMP, earthquake, civil war, etc. This is a far easier way to invade than Putin's current reasoning for invading Ukraine of liberating the Ukrainians from Nazism.
The 2A is great but not very effective against modern tanks, and airpower.
Again, just my opinion, but legalities don't mean squat when weapons are involved. When the USSR fell, Russia agreed to never invade Ukraine if they gave up their nuclear weapons.. Then they invaded Crimea, and now the rest of Ukraine. Kind of stupid move on Ukraine's part back then to give up the nukes if you ask me.
So back to my above statement, the legalities and agreements aren't worth the ink they're written with.
Have you noticed there's no peacekeepers in Afghanistan? How do you think that would play out? Oh sure, they talk the talk, but no country is going to want to go into Afghanistan on a peace keeping mission.. You either cut the military lose and let them conquer, or you stay the hell out.. There's no middle ground..
Peacekeepers only work in 3rd world countries where the citizens have largely been disarmed.. and really, when the rubber meets the road, peacekeeping missions are nothing but NATO trying to fertilize the growth of democracy. Not that such things are bad, but it is what it is.. and it would fail miserably and catastrophically in the USA. There would be blood soaked blue helmets everywhere.. not even their base of operations would survive.
Legal or not, its never going to happen.. our own military wouldn't allow it.