This was the fault of FDR and his fellow New Dealers, who were hostile toward anti-Communist Japan but were very supportive of the Soviet Union and Soviet policy goals for Asia. If FDR and Stalin had not repeatedly pressured Chiang Kaishek not to make peace with Japan, China would not have fallen under Communist control and Manchuria would have remained under Japanese control, and the lives of tens of millions of people would have been saved, not to mention the fact that Korea and Vietnam would not have fallen to Communism either.
In Europe, FDR treacherously prolonged the war by several months and caused the deaths of hundreds of thousands of American and European soldiers and civilians. He spurned every overture made by the German resistance, even though that resistance included numerous high-ranking military officers, including the head of the Abwehr. The German resistance offered to kill Hitler and to return all the lands annexed by Hitler in exchange for peace. FDR refused.
The invasion of Italy would have been unnecessary if FDR had accepted the first Italian peace offer. When Churchill and senior American officials finally convinced FDR to change his mind, it was too late because Hitler moved 24 divisions into Italy shortly before FDR relented. Tens of thousands of American and German soldiers, not to mention thousands of Italian civilians, died in the fighting in Italy because of FDR's atrocious handling of the situation.