I don't have any profound wisdom to offer in response to the breaktaking scenes coming out of Japan (as usual, Andrew Sullivan's Daily Dish is a fount of videos, tweets, and other on-the-scene accounts), but I do want to make one overarching point about unexpected disasters of this sort. One of the reasons that people like me constantly harp on the dangers of overcommitment is the simple fact that much of life remains unpredictable. To quote the distinguished political philosopher Donald Rumsfeld: "Stuff happens."
No matter how carefully you plan, there are always going to be some unpleasant surprises. Maybe it will be an earthquake hitting a longtime ally. Maybe it's an uprising that topples a friendly leader. Or it could be a financial panic, an outbreak of infectious disease, or a military operation that turns out to be much harder than you thought.
When the unexpected occurs, great powers need to have something in reserve. But if a country has already gotten bogged down in lots of costly commitments, and if its citizens don't like to pay taxes and thus tend to underfund anything anyway, it will be a lot harder to respond effectively when a crisis suddenly arrives. Which is why the United States should think harder about its willingness (at least rhetorically) to "pay any price and bear any burden." That's not isolationism; it's just what Walter Lippman called "solvency" (i.e., making sure that our commitments match our interests and our resources, with something left over to deal with emergencies.
All that said, a prompt and generous provision of relief aid to Japan seems like a no-brainer, in sharp contrast to the more vexing question of what to do about Libya.