Stupid question

I know it's obvious, but still I'll speak out my mind following a recent conversation:

So why is Washington pushing so much for a UN-sanctioned resolution against Iran's alleged military nuclear program, when US neglected UN in their single-handedly performed war on Iraq? It turns out UN is not so 'useless' after all...

And also another thought: is Iraq expectedly turning into a second Vietnam, despite all statements about the contrary issued by the Pentagon? And how amusing is this?