We actually have all the power here, and there's one thing we can do to render terrorism ineffective: Refuse to be terrorized.
Stoicism has a lot to say about tragedies like the recent one in Boston. However, I believe the security expert Bruce Schneier says a lot very well, and in a way that speaks to more people than just the Stoic practitioner. So here is his Atlantic article.
Terrorism is a tactic. It is an attempt to shake the minds of those who survive and who observe. Terrorism happens, not in a bomb blast, but in every moment afterwards. It is perpetuated through every newsreel replay that stokes our fear and the worst parts of ourselves.
I was so pleased to see how many people intentionally focused on human kindness today. They gave witness as people helped one another, as thousands of people served one another. Their positive focus is not a denial of how the world works. It's a wide lens view that recognizes the outcome of our innate need for one another; that there are so many caring people in the world.