I don't know that discourse in the public sphere is really the answer. Firstly, foreign policy is almost entirely the President's sole sphere. The legislative and judicial branches of government have very little to do with foreign adventures.

The problem, rather, is the lack of informed discussion and debate in the very private sphere of the oval office - a statement that goes for all administrations, really. In fact, I believe they often do undertake this debate. But the fact remains that military operations always cost more than anticipated and achieve less than advertised. Out of all the US interventions over the years, how much has the world really changed in our favor? Korea ended in stalemate - but that was better than the status quo ante which was a unified (and Communist) Korean peninsula. Vietnam ended in total strategic defeat - and yet constituents in NATO and elsewhere remained convinced of US claims to battle Communism. Just Cause didn't turn Panama into a Jeffersonian paradise, but it did result in a democratic government taking hold. Desert Storm didn't topple the Ba'ath regime in Iraq, but it liberated Kuwait.

Really, what bothers me is the "daddy knows best" routine from those in power. Regardless of whose administration it is, they all pretend to be experts - whitewashing the true risks and overselling the benefits. And so every war we get caught up short by reality.