But where's the serious discussion about the United States totally screwing up and getting in to the conflict in the first place?  Where is that conversation now, to counter these smug neocons authors and bloggers, sipping their lattes and watching their hedge funds some 3,000 miles away from the slaughtering, even as they talk about how (obligatory drumroll) the very fate of Western Civilization is at stake in Iraq?  

American ground troops hadn't been in Cambodia in 1971, so how could our leaving Cambodia have caused the rise of the lunatic Khmer Rouge?

There are historians who argue that (conservative republican) Richard M Nixon's massive bombing of Cambodia that left Cambodian society a bloody, nihilistic wasteland making conditions ripe for the Khmer Rouge takeover. That, would be a failure of your allies on the right side of the aisle, not the left. It was (conservative republican) Jerry   Ford who declined (liberal Democrat) Senator George McGovern's suggestion we should do something to stop the carnage in Cambodia.

Finally  you do not understand the significance of Tet. The Tet Offensive demonstrated that the United States could not win a limited war of attrition. To defend the urban areas, U.S and ARVN forces were withdrawan from the countryside, allowing the communists to increase their control there which was a setback for the pacification effort. Tet also destroyed the South Vietnamese confidene in its own government and South Vietnamese Troop desertion rate went up about 25% after Tet. It showed the NV were prepared to continue a protracted war.

Westmoreland requested more troops from Wheeler. According to our (as in the US) own 7000 page top secret report on Vietnam, Tet was  

A fork in the road had been reached. Now the alternatives stood out in stark reality. To accept and meet General Wheeler's request for troops would mean a total U.S. military commitment to South Vietnam - an Americanization of the war, a callup of reserve forces, vastly increased expenditures. To deny the request for troops, or to attempt to again cut it to a size which could be sustained by the thinly stretched active forces, would just as surely signify that an upper limit to the U.S. military commitment in South Vietnam had been reached

Clark Clifford pointed out to Johnson and McNamara that

"There is a very strange contradiction in what we are saying and doing. On one hand, we are saying that we have known of this build up. We now know the North Vietnamese and Viet Cong launched this type of effort in the cities. We have publicly told the American people that the communist offensive was: (a) not a victory, (b) produced no uprising among the Vietnamese in support of the enemy, and (c) cost the enemy between 2,000 and 5,000 of his combat troops. Now our reaction to all of that is to say that the situation is more dangerous today than it was before all of this. We are saying that we need more troops, that we need more ammunition and that we need to call up the reserves."

Would you like more schoolin' onTet, or have you had enough?