They know the US and the Brits will do nothing militarily...there will be a lot of posturing and the Iranian's will put a big show on...but, again, they will not risk losing the support of the Libs in the US Senate and Congress to defund the wart effort as well as the love of the American press. If they do, even Libs, with their back up against the walls, will fein being enraged by Iran's actions.