America changed more on December 7, 1941, than on any one day in our history.
Stephen Ambrose points out in The Wild Blue:
"The US Army at the beginning of World War II had fewer than 200,000 men (26,000 of them in the Army Air Corps), which meant it ranked 16th in the world, right behind Romania. The Army was pitifully smaller than the millions of men in the Japanese, German, and Italian armies. By June, 1941, the US Army Air Corps had been built up to 1,257 combat planes, nearly all of them inferior to the Japanese Zero, which outnumbered them anyway, and to the German Luftwaffe's fighter fleet, which was four times larger than the American fleet and growing rapidly."
Before December 7, 1941, the US had never had much of a standing Army in peacetime. The Pentagon, just finished to consolidate command and control for the expected war, was intended to be temporary -- a concrete tent to be struck when the war was over and turned into a warehouse as America drew down its military.
We never did.
We've replaced the threat of 1941 with the threat of 1945 with the threat of 2001.
Sixty-five years after Pearl Harbor, America is the world's only superpower. We spend more on defense than the next ten nations combined. And that spending is the largest share of the federal budget.
It's largely because we remember Pearl Harbor.