Not just the US. A lot of countries care. Western Europe (not sure about the east) does as well and will do something even if they aren't as violent as the US. (In fact they are probably going to claim the moral high ground of not assassinating people only because they give evidence to the US and look the other way). South America, South Asia, and Africa will all have at least some helping out, though it isn't clear who will do what.

Most of the blame is going to Russia, though North Korea is a possible source of this, as are a few random countries scattered around. Most stand to lose more than they gain from allowing such crime. (their military might be interested in the ability, but those will be more careful about who they target)