Invading N. Korea and Iran are still within the capabilities of our military. The bigger question is whether we'd still be able to fulfill the other duties of the military - would a further extended military still be able to protect Taiwan from China? Aid Israel? Or the bigger question of "can the US afford this large a military?" I've yet to hear a good answer from the right on those questions.
As for Saddam and WMD's - the assertion that he'd be able to obtain useful WMD's in a reasonable time frame, say 5 years, is laughable at best, given our knowledge know. Hell, given the raw, unspun by the Bush administration intelligance then it's laughable. Especially if we'd concentrated our non-proliferation efforts on Pakistan, who've been selling stuff to half the world it seems.