Without getting into an overall debate about the merits or lack thereof of those wars, what do you suspect might have been the outcome of non-intervention? Admittedly Vietnam eventually fell to the Communists and Iraq has destabilized a lot in the last few years. Still, the failures at the end of either of these wars are not necessarily due to poor decisions at the beginning. Not a little failure comes from political intervention that contributes to a destabilizing that forces on the ground cannot control. Wars are not fought by the military alone. It is a mixture of military and political. In the end, who do we blame?
It is not a question of non-intervention, but a matter of how we choose to engage.
Vietnam fell to the Communists, but none of the predicted consequences of that fall resulted. In fact, just the opposite. Vietnam is now a trading partner. We had the opportunity to side with Ho Chi Minh in Vietnam's civil war. We chose not too. But, doing so would have made a lot of sense because we should have factored in the long-standing adversarial relationship between Vietnam and China.
Many knowledgeable people predicted the kind of instability that currently exists in the Arab world as a consequence of our decision to invade Iraq. And, when you think about it in the context of the history of the region, their arguments made a lot of sense. Yet, those voices were ignored and the people who made such arguments were demonized as un-patriotic.
It's not so much a matter of bad decisions, but a matter of decisions that resulted from bad decision making. War is serious business and the decision to go to war should be deliberative. In the case of both Vietnam and Iraq, the decisions were made to go to war and the only deliberation was that related to making up excuses to justify the decision. So, who do I blame? The administration, congress, and the media and the electorate for failing to hold the administration and congress accountable. I also think the church is at fault for its silence.
As to the concept of "just war," what definition do you prefer?
Saint Augustine's would be a good place to start. In practice, I think the following are applicable:
1. Response to a very real threat.
2. Response to a threat that is not a reaction to our provocation.
3. Armed conflict as a last resort.
4. A response that is proportional to the threat.
Neither Iraq nor Vietnam satisfied any of those criteria.