Impossible to control for obvious reasons eh? In that case the promise was not made in good faith. If you say that you will control immigration when you know damn well that you can't you are telling a bare-faced lie in order to get yourself elected. It's called playing the race card. And the LibDems did not force any compromise on that stated promise. Even the Tories didn't blame them when the policy failed. They simply lied about it, saying that it had been an aspiration, not a "no ifs, no buts" promise. You are defending the indefensible.