From Dan Drezner:
Boston is a tough, resilient town. This sort of thing will shock us in the moment. As shock fades away, what is left is something stronger and more substantive, something that a few homemade bombs cannot destroy. That's the narrative that will hopefully emerge, and it's the one that does the best job of defeating the psychology of terrorism. The next thing that will happen is foolish, uninformed speculation about who or whom was responsible.