Stephen M. Walt in Foreign Policy:
To begin with, any romantic partnership is essentially an alliance, and alliances are a core concept on international relations. Alliances bring many benefits to the members (or else why would we form them?) but as we also know, they sometimes reflect irrational passions and inevitably limit each member's autonomy. Many IR theorists believe that institutionalizing an alliance makes it more effective and enduring, but that’s also why making a relationship more formal is a significant step that needs to be carefully considered.
Of course, IR theorists have also warned that allies face the twin dangers of abandonment and entrapment: the more we fear that our partners might leave us in the lurch (abandonment), the more likely we are to let them drag us into obligations that we didn't originally foresee (entrapment). When you find yourself gamely attending your partner’s high school reunion or traveling to your in-laws for Thanksgiving dinner every single year, you’ll know what I mean.
Realists have long argued that bipolar systems are the most stable. So if any of you lovers out there are thinking of adding more major actors to the system, please reconsider. As most of us eventually learn, trying to juggle romantic relationships in a multi-polar setting usually leads to crises, and sometimes to open warfare. It's certainly not good for alliance stability.
More here. [Thanks to Kris Kotarski.]