The events in London today are horrible. On the bright side — if there is one — Londoners seem to have handled the crisis extraordinarily well. The best we can do, as Americans and individuals, is what we would do with any friend, let them know that we are saddened by the tragedy (a word that is overused, but appropriate in this case), and that we stand by to help in whatever way they need us.
As to our “reaction,” Tim Bray says it better than I can. I almost copied over his short post wholesale. Basically, the best response to something like this is to figure out why it happens, and try to cut it off at the source.
My suspicion is that one of the best ways of reducing such violence is to work on spreading literacy and learning here in the US. Enlightenment is contagious, and a population at home that mixes tolerance and skepticism is likely to provide a better model than one that sees violence as the most effective means of solving disputes.
As Mr. Bray notes, this is not an excuse for such horrible acts. I think we can agree to despise such violence, and then move on to examine effective ways of reducing its future occurrence.