Step by step years ago I had a former officer who tought a history course on Vietnam show how Diem was very brutal, but no more so then the North Vietnamese, but he was also a very competent strategist who knew what South Vietnam needed to do to win the war. He was willing and planning to use South Vietnamese troops to invade Laos as he knew he could not win while endless supplies and troops got down to the insurgents. That defied our policy so we got rid of him.
The officer who fought there outlined step by step what we did wrong in Indochina and how we could have done things much better. And, not all of this is what we could have done in Vietnam. At home he believes Johnson had to decide between the War and his Great Society and he had to mobilize the country and put up war time controls of the press like was done in Korea so that Americans didn't watch dead Americans on the news each night.
He said America had many good generals at the time better then Westmoreland and even Creighton Williams who really understood how to deal with complex partisan conflicts, but they were passed over. He made clear how winnable the war was, but JFK and Johnson had to not just do things differently, but treat it as a real war and not just a limited helping hand police action we were half in and half out of even when we had hundreds of thousands of troops on the ground Johnson was still sending signals he was half hearted about his commitment as others have said.
In order to bring South Vietnam to the peace table by 1967 which was entirely possible the WH needed to treat it like a real war they were invested in winning, not a conflict they were invested in not losing. To be tongue in cheek about it quoting Paula Broadwell's book we needed to be 'all in' or not in at all when it comes to getting involved in a land war in South East Asia.
Nixon still could have pulled things out the officer said and was presented with seven options for Vietnam by order of how much he wanted the U.S. to do in Vietnam. The first option was an 'all in' option, the next was less 'in', and on and on to the bottom and Nixon choose the bottom option as it was the politically easiest route and still South Vietnam's Army with U.S. air support and almost no ground troops managed to beat a full invasion of the South in 1972 and it took them three years to be able to mount another one and at that point in time the Viet Cong was a spent force. But, by 1975 the U.S. politically wasn't willing to provide air support or real economic support to the South anymore while the North's allies were which is why the 1975 conventional invasion of the South by North Vietnam succeeded where the 1972 invasion failed.