As Glenn Snyder and then Patricia Weitsman so clearly identified (as well as Jack Snyder and Thomas Christensen), alliances always pose two threats to those who join: that they might be abandoned despite reassures to the contrary AND that they might get dragged into a war they do not want to fight. So, abandonment vs. entrapment are the dual horns of the alliance dilemma. So very relevant today as the story du jour is of "Decoupling" South Korea from the US.
It has long been a concern that North Korea would maneuver to separate the US from South Korea, and they have made a heap of progress lately. This morning there are reports of talks leading to more negotiations between North and South Korea. The concern is that South Korea might sign some kind of separate peace, giving into North Korea and leaving the US out.
For much of the cold war and post-Cold War period, the South Korean concern would be that the US would abandon it. Jimmy Carter proposed and then reversed himself on pulling the US troops out, as this undermined the credibility of the US commitment to South Korea, for instance. Just like the Europeans, the South Koreans worried that the US might not show up, that it would be very costly for the US to defend South Korea. The advent of the North Korean ICBM with the capability to hit the US gave the South Koreans the chance to re-visit all of the "would the US sacrifice Chicago for Bonn or Paris" debates. So, yes, the South Koreans still fear abandonment and perhaps even more so with an uncertainty engine in the White House who confuses North and South. You would think that this fear would lead them to focus on tightening the alliance, not decoupling.
Ah, but here's where the other horn of the alliance dilemma gets super-pointy: the US has been making noises about a new Korean war, that a punch in the nose or whatever effort to disarm North Korea, would be harmful to South Korea but not the US. Indeed, Lindsay Graham has recently agreed with the White House rhetoric of, well, burning South Korea to save American lives. The South Koreans know only too well that the opening shots of a new Korean war would lead to hundreds of thousands, if not millions, of dead South Koreans. So, with all of the war talk in DC, they have good reason to fear being entrapped in a war they do not want.
So, I can't blame the South Koreans for seeking an alternative path, away from the races up the escalation ladders. The problem, of course, is that if South Korea is decoupled from the US, that the alliance is broken, then North Korea can break its promises to South Korea and the US will find it hard to respond to a fait accompli in the aftermath of US forces being expelled from South Korea (the possible result of decoupling). Where does Japan fit in all of this? Completely screwed but that is an alliance that will be broken via other means (trade wars)....
In sum, Trump has made a difficult problem far more challenging. The no good policy options problem is now far more likely to be all the worst policy problem. Thanks, Trump.