Those who say the bigger you make it, the less elite, are correct. That said, the training we have for our active duty military is probably at its zenith right now: we have a combination of lessons learned, combat hardened NCOs and officers, and standards that have continued to evolve and improve. And that will never be enough to avoid the horrors of war.
In Korean War v.2, we would commit nearly all of our forces. It's true that the 38,000 U.S. troops in South Korea, and the Marines, Sailors, and Airmen in and near Japan deter aggression on the North Korean peninsula. The more than 500,000 South Korean servicemembers do, too. And if North Korea's government convinced itself that the U.S. was unprepared to respond, it is very easy to see a desperately poor, but militarily dense, country think that the solution to its ills is to invade the wealthy country immediately south of it. As long as the Chinese stayed out of it, the U.S. and its allies would eventually win, but we would likely have to send in our reserves, as well as most of our active component.
Also, as noted above, the United States hasn't been in the military business of wiping out a country since it dropped the second atomic bomb on Nagasaki. Even then, with notable exceptions, we tried to preserve the German people, but not so much the Japanese. I think our attitudes on race have evolved since then. Nowadays, we have little appetite for "total war," in which we obliterate people along with their governments. It runs counter to our "We the People" ethic to destroy people who live under autocratic control. But that makes our need for ground troops even higher, for the reasons CD and CWS point out.
That said, I wonder whether more troops could have made the difference in Iraq in 2003. Possibly. That war turned out very much like I expected it to at the time: militarily the Iraqis were no match for the U.S., but culturally, we were near completely unprepared to make a lasting peace in which we came out the victor. Perhaps that is a vestige of a country (and region) largely constructed from the ruins misguided colonial (primarily British) thinking. Would we have been better positioned to manage this in 1991? I doubt it, but maybe the extra ground troops and a stronger middle class in Iraq, not yet further deteriorated by a decade of sanctions, would have helped.
Many of the problems we have faced over the last twenty or thirty years have been problems of our own making--at least partially. But it is also possible that those problems were better scenarios than had we not intervened in the ways we did. Most of our international policy post WWII was intended to prevent the spread of the communist empire. It was largely successful, and constraining that empire helped it die faster than it otherwise would have. That may well have been worth all the difficulties we caused ourselves. Undoubtedly mistakes were made, and with the benefit of hindsight, we could have done things better. That is not the way the world works.
Could we have reached the Iranian government in 1952-53 and made peace between it and the UK regarding oil production, thus making Iran our friend in the middle east? Could that have been a better check on Soviet efforts in the middle east? Probably. Could we have understood the massive problems French colonialism caused in Vietnam, and supported a democratically elected government that represented the people of that country, instead of backing failed regimes? Maybe.
And what are we doing now that with the benefit of hindsight we could do better? Probably a lot.
I disagree with TR Fehrenbach's more militaristic tendencies. He over blames military problems on civilians. But I agree with his theory that the U.S.'s willingness to fight proxy wars to hold back communist expansion prevented more of that expansion, and I think that made the world a safer place. Of course, he wrote This Kind of War before the Vietnam War. Much more of a failure than Korea, it is just as possible that the U.S.'s willingness to fight and lose Vietnam over the course of a decade continued to show the Soviet Union our willingness to match and check their attempts at expansion. At the same time, the Soviet experience in Europe (outside of Russia) created its own drain on the Soviet machine, and probably did much to check Soviet expansionism. Keeping all of those people under the government thumb is difficult and costly.
Back to babbling...