[ShadoWarrior]

Oddly enough, and very fortunate for the UK (and the rest of the world), Germany failed to learn the lesson of their own success with U-boats in WW1.

When World War II started, Germany only had 65 U-boats, with just 21 of those at sea, ready for war. During the war the Germans sank 5,150 allied ships displacing 21.57 million tons. Of this, the U-boats were responsible for 2828 ships of 14.69 million tons. To place this in perspective, the Germans sank the equivalent of the entire British merchant fleet at the start of the war. Additionally, submarines destroyed 187 warships, including 6 aircraft carriers and 2 battleships.

WW2 U-boat production:

1935 (14)
1936 (21)
1937 (1)
1938 (9)
1939 (18)
1940 (50)
1941 (199)
1942 (237)
1943 (284)
1944 (229)
1945 (91)

Total: 1153

Imagine what would have happened if Germany had used a different Z-plan, one in which resource-intensive dinosaurs like the cruisers and battleships whose keels were laid down prior to the outbreak of war had been deferred to later in the plan in favor of truly massive U-boat construction. How many U-boats could Germany have built in place of the Bismark and Tirpitz? As it was, Germany came close to bringing England to its knees, even with a late start at ramping up U-boat production. Let's suppose that Germany had built 200 U-boats in 1939 and another 200 in 1940. (That's moving the historical production ahead by just 2 years, which is not unreasonable.) Germany succeeds in forcing England to sue for peace by the Spring of 1941. With no threat in the Mediterranean from England, Germany has no need to intervene on Italy's behalf in Greece. The west flank is secure and Operation Barbarossa can begin on its original schedule and with more power as Germany no longer needs to tie up large numbers of men, tanks, and planes in the west. Six weeks more time allows Germany to take Moscow long before the fall rains turn everything into a muddy morass. Leningrad falls soon thereafter. Even if Stalin doesn't sue for peace (which is unlikely, and it's equally unlikely that Hitler would have accepted even if Stalin offered), by the time the Siberians arrive they'd find the Germans already hunkered down in Moscow and Leningrad. Their counteroffensive would have limited effect. With reduced winter losses, and starting positions further east, Germany is more likely to succeed in the Caucasus operations of 1942. If they capture the oilfields (likely in this alternate history), that's pretty much it for any chances of the Soviets ever being able to throw the Germans back on the defensive. Best case for the Russians is that things settle into years of bloody stalemate on the Eastern front. Worst case? Japan attacks a fatally weakened Russia in late 1942 and it's the Russians that have to fight a two-front war, and do so without any lend-lease from the UK or the US.

And speaking of the US, with England at peace with Germany, the Germans never declare war on the US. The French are FUBAR. There will never be a "second front". The US concentrates all its might on Japan. The Pacific war probably ends in late 1944 or early 1945, and without atomic weapons, which are not fast-tracked into development because the US is never threatened by Germany. The invasion of Japan is horrendously bloody.

Germany completes the Final Solution. Germany also, eventually, either gets Turkey to join the Axis, or conquers it. After which Germany sweeps through the Middle East, swallowing up Arabia and Persia.

Two superpowers arise from WW2: the US and Germany, with the Third Reich as the largest empire the world has ever known, having conquered nearly all of Europe, and a substantial portion of Asia.