I'd say that I don't think they can because everyone is going to want hard proof after the Iraq debacle, but they do what they want. I think Iran needs to be wiped off the map eventually, just not now.
Actually -
Afghanistan - The United States NEVER provided the world with proof that Bin laden was behind 911. Condi went on Meet the Press and said a 'white paper' would be out shortly, telling the world why we were invading. it never came.
Iraq - The United States never had proof of WMD. UN team / hans blix was in there, and didn't find a thing. We promised we'd provide evidence and we never did.
Iran - The United States has "classified info" they can't release. But they do assure us it will come out eventually. Um, the last two wars show we'll never see it.