There were many turning points to this war. Let me list a few:
- The German invasion of Russia in June of 1941. This opened the Eastern Front, and that is something that would eventually lead to Germany's defeat.
- The attack on Pearl Harbor in December of 1941. This got the US into the war, changing the whole face of the war.
- Stalingrad, which destroyed much of the German army and ensured that Russia would not fall.
- Midway, where much of the Japanese fleet was destroyed. After this, the Japanese were on the defensive for the rest of the war.
These are perhaps the biggest turning points in the war.
There were many turning points, in addition to the ones mentioned above.
In North Africa, Rommel's Afrika Korps never recovered from losses at El Alamein in November 1942, and directly led to Hitler's loss of all of North Africa. This allowed the invasion of Sicily and mainland Italy in 1943.
Another turning point was in the Battle of the Atlantic, which pitted German U-boats against Allied shipping and convoy escorts. The invention of SONAR and ASDIC, and allowed the men and material to get through which made D-Day and a sustained invasion of Europe possible.
I would agree with post #3 that the Germans defeat at Stalingrad and then again later at Kursk gave the Russians much needed initiative to go on the offensive against the German's. The Germans also suffered huge casualties that they were unable to replace.
I would have to say the successful invasion on D-Day was the turning point in the war. Hitler was aware that his days were numbered if a successful invasion came from the west. Had he prevented the invasion, his hold on Western Europe would have remained firm.
I would have to go for Pearl Harbour as being one of the most significant events of WWII. The way that this triggered American involvement on the side of the Allies was crucial in winning the war. Pearl Harbour, ironically, was one of the best things that had ever happened to the allies during wartime.
The major turning point of the War in Europe probably was the Battle of Stalingrad. The major German advance was stopped on December 6, 1941, one day before the bombing of Pearl Harbor. From that point forward, Germany was forced to fight a defensive war. The second most important European battle would have been Operation Overlord, the Normandy Invasion, commanded by General and later President Dwight D. Eisenhower, which established an allied beachhead in western Europe and allowed the Allies to advance on Germany from two sides, East and West.
In the Pacific, the two major battles were (1) the Battle of Coral Sea, which stopped the Japanese advance in the Pacific. The Battle is noteworthy because it was primarily fought by carrier borne aircraft. The two opposing fleets never caught sight of each other. (2) The Battle of Midway Island, which is considered the turning point in the Pacific War. General Isoroku Yamamoto had planned to intercept and destroy the U.S. Pacific Fleet when it arrived there. As it happened, the U.S. had cracked the Japanese code, and knew the plan ahead of time. When Yamamoto's troops arrived, the Americans were waiting for them. This battle marked the turning of the tide in favor of the Allies in the Pacific.