Dönitz's decisive U-boat fleet would provoke an Allied answer

How many U-boats would Dönitz have needed to make a German victory possible in the Atlantic war?

His desired fleet of roughly one thousand boats might sustain one to two hundred on patrol and seriously threaten Allied shipping, but building it early would sacrifice other German forces and prompt Britain to order more sloops and escorts.

A much larger U-boat arm available at the start of the war could increase the pressure on Atlantic convoys. Yet Germany would pay for it in tanks, aircraft, or surface ships, while the visible buildup gives the Royal Navy time to expand escort construction outside treaty limits. Fewer German surface threats would also free British capital ships and carriers for the Mediterranean and Far East, so the counterfactual changes Allied allocation as well as German striking power.