The US was never a 100% free market. But it was pretty close, and the beauty of a free market is the closer it is, the better it works.
> anyone who thinks a system that observes slavery qualifies as a free market should be embarrassed.
I usually say "excluding the slave south". Nevertheless, let's compare the slave south with the free north. The free north economically buried the slave south, which was a major cause of the southern rebellion. They wanted to protect their economy from the depredations of free labor.
The south lost the war because the north buried them with arms and supplies and trains and everything needed in plentiful supply to destroy the southern barefoot armies.
A similar thing happened in both WW1 and WW2. The American free market decided the fate of Germany and Japan the day the US entered the war, because Germany and Japan simply were overwhelmed. The US supplied not only themselves, but supplied Britain, France, and Russia, and still buried Japan with overwhelming endless supplies of everything the military wanted.
The Japanese understood this (Hitler didn't), and tried to win WW2 with one stroke (Pearl Harbor). When that failed, they put everything on the line at Midway. The carriers were sunk, their best pilots were killed, the planes were lost, and Japan could not replace them, and everything went downhill for them from there. The US lost carriers and airplanes and just built lots more.
Hitler lost WW2 the day he declared war on the US, in probably the stupidest military move in history. You can't win against a free market country unless yours is free market, too.