The effectiveness of any nation in war is not just its succesful prosecution but wether the objectives were fulfilled in the longer term.
WWI is a classic case. Millions died and Germany was defeated but the objective as far as Britain was concerned was to eliminate a potential threat to its world power, hence the arms race in the years preceding that war.
Despite emerging victorious the true victors were the US. Britain went into a long decline ,and Germany, far from being less threatening, became a much greater problem and there is a pretty strong case to be made that the Treaty of Versailles was instrumental in that.
Another measure ,I would have thought, is the percentage of troops killed or wounded.
During the US civil war the North deployed far more resources, and even if you concede that it lost more men, the percentage losses were far less .
The US was able to develop its industry before it became involved in WWII by supplying the allies with materiele. This gave it a massive technical boost.By the time an effective US force had been deployed the Germans were already being driven back by the Russians whose contribution cannot be overstated.
Courage and ability can make a huge differance but faced with a massive technical and industrial superiority any nation trying to take on the US would be very foolish.
There are other ways of winning a war too, such as economic warfare.The Eastern bloc and China used proxy wars and even won in Vietnam, but the the time that US troops bought with their lives allowed the economic might of the US to crush all its opponents.
So if ,in evaluating wether the US military are that good, you take into account that it has enabled the US to dominate the latter half of the 20thC you would be forced to conclude that it really is *that good[/].