What lesson did the US learn at Saipan?

While a decisive battle in the Pacific for the United States, they did learn just how determined and willing the Japanese were to continue fighting until the very end. On top of that, many of the defeated Japanese forces committed suicide in the caves of the island while many of the native civilians committed suicide as the Marines were advancing to the north of the island after Emperor Hirohito gave an imperial order that encouraged civilians to do so.