What "they" believed is going to depend heavily on who "they" are. I'm not aware of any polling data or other sources that would reveal what the general American public thought. But I doubt that any informed observer would be so cocksure.

Prior to the late 20th century, it had been something of a United States tradition to badly neglect the armed forces except in time of war, and after the termination of war, to prune it back to the smallest size possible. This was true after the Revolution, the War of 1812, the Civil War, WWI, and it was true of WWII as well. Following the successful conclusion of that conflict, the United States massively reduced the size of the armed forces and cut military spending - especially Army spending - to the bone. While the Navy and the Air Force (the latter especially) remained reasonably potent, the US Army in 1950 was a shell of its former self. From a wartime peak of 89 combat divisions and 6,000,000 men (not counting the Army Air Force, which numbered another 2,200,000), the Army had been reduced to 593,000 men and ten combat divisions by 1950. What's more, those divisions were weak. They were chronically understrength - that is, lacking men and equipment - and due to budget cuts, most of the equipment they did possess was WWII-vintage, much of it worn-out or poorly maintained. Training suffered, material conditions were far from ideal, and morale among enlisted men was commonly poor. Omar Bradley, then-Army Chief of Staff, wrote that the US Army of 1948 was "in a shockingly deplorable state" and possessed “almost no combat effectiveness and could not fight its way out of a paper bag.” Any competent military observer could see that this was not an army that was prepared for war. It was committed to war out of desperation, rather than arrogance.