What was the most decisive battle ?

I would go for THE most important. If Britain had lost, it wouldn't just be them that were gone. The US would not have joined the war as fast, if ever.

Not to mention a friendly port would have been no where in sight for an effective Europe landing

Juan Bertin wrote:

In summer 1941, the Germans could have made a decisive advance on Moscow. The defenders were so ill-equipped after 3 weeks of war, some of them went into battle with no weapons. Russia already had contigency plans to continue the war from Siberia. If Moscow would have fallen in 1941, Hitler would have had the initiative for a long time. Hitler was hesitant to attack Moscow for reasons not entirely clear to me, but he didn't like Napoleon's capture of that city in 1812.

PS: My English is not perfect.

In summer 1941, Army Group Center had a window of opportunity to take Moscow, the Russian defenses on the Minsk-Moscow highway were disorganized. By September, the Germans counted nine Soviet reserve armies, which discouraged Hitler and his Generals, though they kept pressing forward. By the time the snow was falling, there was no doubt the Wehrmacht was in for trouble. That winter was so cold,even retreat seemed unappealing, so the Germans fought from whatever shelter they could find. The Germans had planned to take Moscow in an encirclement battle which would have been fierce, and Germany would have won, if they had started at the right time.Hitler declared war on the United States a few days after Pearl Harbor and Churchill remarked "So we have won after all."

You've just said that in your post above?

Do we really wanna put Pearl harbour there?

either way thats the most decisive battle on the list.

If Churchill said something the world would be a lot different

the battle of Britain, Stalingrad, and Moscow all were very important in Europe! But they were not important at all in the war in the pacific. A single battle, pearl harbor, led several to important battles in Europe and Africa, and nearly every major battle in the pacific.

Well, if Britain lost the battle of Britain then the Germans would have probably have conquered Russia within the year. Meaning they had easy access to Japan and the Pacific battlefield. Work on the German surface fleet would have resumed and they could have sent their powerful battleships and aircraft carriers to help the Japanese, not only their own surface fleet but the Royal Navy's huge inventory would have been available to them meaning an even bigger numerical advantage over the Americans. But of course, let's not forget the dreaded U-boats that would have destroyed American shipping.

The German Heer would have easily beaten an inexperienced American army and the luftwaffe was full of hardened veterans with months of combat experience, maybe even years.

With Roosevelt knowing all this I think he would have come to some sort of agreement with Hitler and Japan so that he didn't go to war with the two most powerful countries in the world.

Truly, in the Pacific, it was not Pearl Harbor, but Midway that was the turning point. The Americans broke the Japanese military cipher and knew that they would attack the Aleutian Islands in Alaska to lure the remains of the Pacific Fleet up there, then attack Midway to lure them down to their destruction. What Japan didn't know was that all no American Carriers were damaged at Pearl Harbor and that most of the ships they did sink there were repaired and refloated. When the Japanese did attack Midway, they were met by the aircraft from Midway, Enterprise, Hornet, and Yorktown, who was coming up from behind the other 2 carriers. The Japanese attack was repelled and the American counter-attack sunk almost all the Japanese carriers in the fleet, for the loss of the Yorktown. (3 out of 4 Japanese carriers were sunk, correct me if I'm mistaken)

This turned the tide against Japan and allowed America to pull their eyes from the Pacific and focus on Europe and the Atlantic. This would lead into the planning of Operation Overlord (D-Day) and the research from ways into a way to eliminate the U-Boat "Wolfpack" in the Atlantic.

In Europe, Stalingrad is the decisive battle, the Russian Winter, the high-causalities, and Soviet counter-attack after the Germans had taken the city, resulted in a large portion of the German Army captured or killed. It would end the German offensive into the USSR, and be the beginning of the end of The Third Reich.

All four Japanese Carriers were sunk, It might have been three but when the Yorktown was hit the first time the damage was repaired and it looked fine. The Japs returned and found the Yorktown. Again they attacked thinking it was a different carrier because of how quickly it was repaired. The Americans found the remaining Japanese carrier and sunk it.

Stalingrad was important morally for Hitler and his generals (and armies); it showed that the Russians were ready to sacrifice themselves in the hundreds of thousands to deny the Germans the city. Pearl Harbour brought the US into the war, sealing the Axis' fate.