Once Iran - any belligerent country really - has nukes, going to war with them becomes EXTREMELY difficult. That’s why it is (was) so important to prevent Iran from getting nukes in the first place.
Yes, to avoid the disaster of going to war with Iran after they get nukes, there has always been the possibility of going to war with them before they actually get to that threshold to prevent them from reaching it - Obama has always claimed the main reason he wanted his super deal was to avoid that “alternative” - but now to induce brain-dead senators like Casey of Pennsylvania to support the deal he is promising that if Iran violates its terms, he will go to war with them immediately to stop them from moving on to nukes - making the agreement, which Iran is already breaking by sending people to Russia to seek missiles for example, meaningless.....