Are you referring to the US in the 1950s, as tellers of this particular just-so story usually do? If so, that prosperity came about because we had just finished shooting, bombing, nuking, and in general pounding the living daylights out of the rest of the industrialized world.
WWII wasn't about who was right, it was about who was left. To the victor went the spoils. Meanwhile, there was a war to pay for, hence the high tax rates.