Iran is making noises about a pre-emptive attack on American forces in Iraq. They claim that this would turn the US presence in Iraq from an advantage against Iran into a problem, as they would become “hostage[s].” This is about the stupidest thing Iran could do at this point. There is no guarantee that the US will attack Iran, but it makes it a guaranteed thing if they attack our troops. Not only this, but if they strike into Iraq, Iraq could very well declare war against Iran themselves — not likely, but always a possibility. The threat against Israel is much more direct, and more likely to cause problems. Iran will be very careful about how they handle the US, but may be less careful about attacking Israel. This would also be stupid as Israel has nuclear weapons, means to deliver them, and a much MUCH shorter fuse than the American public has.
In all, likely just to be hot air to keep pressure on Washington, but it does show just how far they have to stretch logic to make threats.