What was the worst mistake [you pick it] made in WWII?

Common question on Quora.  The worst mistake Japan made in WWII was attacking Pearl Harbor.  Prior to Pearl Harbor America was deep into isolationism, the idea that we could stand proud here in North America while the rest of the world sank into chaos.  Isolationism built on the unsatisfactory outcome of WWI and claimed that all we got out of WWI was profits for arms manufacturers (merchants of death they were called).  Japan had been agressing against China, and was running Korea and Manchuria as colonies.  We did not approve, and we had sent a lot of diplomatic nastygrams to Japan.  We finally decided to stop selling crude oil and scrap iron to Japan.  The Japanese could have replaced American sources of supply with oil from the Dutch East Indies, and scrap metal from somewhere.  The Germans had invaded and occupied the Netherlands, the Dutch colonies were on their own.  Should a Japanese task force conveyed a few Japanese bankers and their check books to the Dutch East Indies the Japanese could have acquired all the oil they needed.  We would have sent them a few more diplomatic nastygrams, but there was no way we were going to intervene militarily.  Japan could have done pretty much anything they pleased in Asia so long as they didn't attack American territory.
   After Japan sank our battle fleet at Pearl Harbor isolationism vanished, poof, within a few hours.  We were pissed off.  We had a far larger population than Japan, we had a far larger industrial base, we were a continental power, self sufficient in just about everything.  And we were mad. Japanese Admiral Yamamoto said at the time "I fear we have awoken a sleeping giant and filled him with a terrible resolve."  He had that right. 
   As it was, the Pearl Harbor attack changed the course of WWII.  We got our act together and clobbered both the Nazis and Japan.