It began a week ago IIRC, when Putin filed a 'concern' at the United Nations about shelling in East Ukraine. I've been saying for the whole situation I thought Putin is likely to invade, with that only increasing to nearly 100%. I listed some indicators as they happened, such as his evacuating his super-yacht from NATO and rescheduling nuclear missile tests, and there are many more.
Our trigger should be as soon as the 'peacekeepers' cross the border without permission, the war starts, and our complete sanctions start.
While Putin is very wrong here, and war is not the time for a good discussion of this, as I understand Ukraine is split in two between the pro-Russia side and the pro-west side, and there's long been an issue what to do about that.
I wonder if having the pro-Russian side move to Russia might not have some benefits, stabilizing Ukraine into a more unified state, and it's better than Putin entering the pro-west side of the country and overthrowing the government, though getting there by Putin's use of force is not the way to do it.
People might want to make analogies to Hitler's taking of the Sudetenland in Czechoslovakia, and the history of the false peace by agreeing to it, but as I understand that regions actually did prefer to be part of Germany, and there's no reason but Hitler's choices and agenda why that might not have been a reasonable plan. Not every concession need be treated as exactly the same 'appeasement'.
Unfortunately, such changes should be discussed in a context of peace and justice and not under conditions of war, and even doing the right thing can be ill-advised when it rewards wrong.
These situations, unfortunately tend to be dominated by power over justice, selfishness over justice, and people like to simplify them to the point of black and white. So that people have difficulty recognizing 'here's where they're wrong' (long list for Russia), and 'here's where we might be wrong', not a short list either.
Face-saving pursuit of justice is a lot easier when the use of force hasn't changed the issue to the need to punish aggression, and having concessions look 'weak'. Wars can be fought over 'appearances' sometimes. Putin's warmaking and treachery has stabbed peace in the heart - but I think we should not forget that peaceful avenues might not have been opened by us at times, as well.
Remember how well we defended 'every country's right to choose its own alliances' when Cuba chose to ally with the USSR. Putin would have to launch dozens of assassination attempts and years of covert terrorism in Ukraine as WELL as an invasion just to match us. Two wrongs don't make a right and we need to oppose his aggression, but that might not be the whole story, also.