Memorandums have less prominence than treaties so the public relations cost to ignoring them is indeed smaller.
When something doesn't oblige one to do something, and everyone understands that well in advance, then yes, the PR hit from not doing that something is indeed small... You gain a reputation as a promise-breaker by breaking promises.
Yes, he would certainly do something symbolic, but would he take military action against Russia if Russia, say, decided to take back Estonia?
He has to, or else the US empire collapses worldwide: the US holds very few territories outright, it depends on host countries like Germany, Japan, and South Korea, who generally have defense clauses just like NATO and allow & subsidize the US bases in part to benefit from mutual defense clauses. If a NATO country is invaded without a real defense, then America's credibility goes up in smoke. The day after the invasion, just in East Asia: SK restarts its nuke program, NK begins extorting more from SK under the threat of invasion, Japan begins a covert nuke program and begins the process of expelling the US from Okinawa (a long-running sore in their domestic politics justifiable only as part of the US nuclear umbrella, and a solution which costs the US much of its capabilities against China), and so on and so forth.
If Ukraine were a NATO member and Russia still did what she did, do you think that the U.S. would have taken military action against Russia?
Oh yes. And that's in part why Ukraine was never allowed to join NATO: too close to Russia.
He has to, or else the US empire collapses worldwide:
Lots of leftwing intellectuals would love to see the U.S. empire collapse. We don't know Obama's opinion on the topic because he would be smart enough to hide any such anti-patriotic views.
But I doubt that letting Russia take a small NATO country would cause the collapse of U.S. power abroad. Paradoxically, it might increase our power as nations put more effort into pleasing us and begging us to station troops on their soil to act as tripwires.
You are right that Russia taking Estonia would cause lot...
Some of the comments on the link by James_Miller exactly six months ago provided very specific estimates of how the events might turn out:
James_Miller:
Me:
"Russians intervening militarily" could be anything from posturing to weapon shipments to a surgical strike to a Czechoslovakia-style tank-roll or Afghanistan invasion. My guess that the odds of the latter is below 5%.
A bet between James_Miller and solipsist:
I will bet you $20 U.S. (mine) vs $100 (yours) that Russian tanks will be involved in combat in the Ukraine within 60 days. So in 60 days I will pay you $20 if I lose the bet, but you pay me $100 if I win.
While it is hard to do any meaningful calibration based on a single event, there must be lessons to learn from it. Given that Russian armored columns are said to capture key Ukrainian towns today, the first part of James_Miller's prediction has come true, even if it took 3 times longer than he estimated.
Note that even the most pessimistic person in that conversation (James) was probably too optimistic. My estimate of 5% appears way too low in retrospect, and I would probably bump it to 50% for a similar event in the future.
Now, given that the first prediction came true, how would one reevaluate the odds of the two further escalations he listed? I still feel that there is no way there will be a "conventional battle" between Russia and NATO, but having just been proven wrong makes me doubt my assumptions. If anything, maybe I should give more weight to what James_Miller (or at least Dan Carlin) has to say on the issue. And if I had any skin in the game, I would probably be even more cautious.