So whats wrong with Nuclear Iran, big deal?

Israel has 400 nuclear weapons, i am sure they can defend themselves, they have been able to up till now(although with some help). So whats the problem? Anyway, if they duke it out, then America has less things to worry about like wars and terrorists and vetoing everything in the UN as favours for Israel. So whats there to worry about? Please no zombie answers like "Israel is our ally blah blah, i heard it on Tv"