Actually, I think the US military is effective, the people behind the military (politicians, citizens) usually are not. Winning isn't difficult, what is difficult is stabilization afterward which Americans seem to not want to participate in.
Vietnam could have been a win for the US if they didn't leave as abruptly as "we" did. There was actually some stabilization in South Vietnam, and its own military had over a million soldiers while North Vietnam had quite a bit less than that but Americans had enough of the war, the loss of life, etc., and wanted out, and so Nixon ended it and South Vietnam fell apart because they still needed the backing of the US. Look at North Korea, we're still there but we left Vietnam.
Americans (for the most part) like to believe they won WWII, but, there were many other countries already fighting, and all the US did was help shorten the war, especially with Japan by using atomic bombs. The war with Japan, from what I've read over the years, would have carried on til about 1947-1950 -- so, was it really that necessary to bomb them? Most say "yes", few say "no".
The US has an effective military, it has an ineffective country.