I find it easier to identify the wars that were LEAST effective. These would be the wars in which the USA was fighting enemy forces that were trained and supplied by China: the Korean War and the Vietnam War.
I can accept the prevailing point of view that we had to stop China from simply annexing Korea. But since Korea is attached to China, we were basically fighting China in its own backyard. We managed to keep them from annexing South Korea, but what cost? And what did we gain for all those dead American soldiers?
Twenty years later... we got Hyundais! Whoopee!
I will (grudgingly) forgive the U.S. government for sending troops to Korea. But I will NEVER forgive them for sending troops to VIETNAM! This was a replay of Korea: making war against a country that is adjacent to China.
The Chinese had learned a lot about us during Korea, so they beat the pants off of us in Vietnam. There was no armistice; our troops had a few hours to board a fleet of helicopters and get the hell out of the country before the Chinese troops arrived.
This would have been bad enough if our purpose for attacking Vietnam was to "save" the population from communism. But that wasn't it at all. Various corporations had been quietly lobbying our government to start that war, because war is good for business.
The aerospace industry obviously made a fortune by building military aircraft--and every time one of them was shot down, they got an order to build NEW ONE! The big shots were making a fortune off of OUR DEAD PILOTS!
The chemical industry also made a fortune, although it wasn't so obvious. Our brilliant strategists felt that the Vietnamese countryside had too many trees, which made it easy for people to hide with anti-aircraft weapons. So Dow, Monsanto and a couple of other despicable chemical companies developed heavy-duty herbicides that could turn a lovely forest into a barren acre of dead trees within a couple of days. These chemicals were so strong that the forests could not revive for TWO OR THREE YEARS! They had actually POISONED THE SOIL! The Vietnamese were a rural people who ate fruits and vegetables they gathered from the forests, augmented by what they could grow in their own farms. With their soil poisoned, they were STARVING!
The third industry to make a fortune from the Vietnam war was a brand new one: the software industry. Third-generation mainframe computers had just been invented, giving any organization a tremendous push forward in the way it managed information--such as blueprints, plans and charts. I worked in information technology (or I.T., as it was quickly shortened), but in a municipal government, so I personally didn't get rich from the war. But I had several friends who had been recruited by start-up firms and were happily working overtime--writing programs that were to be used to manage the war, although many of them were never told exactly what they were doing. This was, after all, the era of the Counterculture, and the majority of people in their 20s would have barfed if told they were supporting a war.
Aerospace, chemical and software: three growing, powerful industries were making a fortune off the war, and the beauty of it (from their amoral perspective) was that it didn't matter at all whether the U.S. would win or lose!