Although the U-boats succeeded in sinking both battleships and merchant vessels during World War I, it wasn’t until the early years of WWII that they really came into their own. While the Treaty of Versailles had placed limits on the number of ships that the German Navy could maintain, there was no such restriction on undersea vessels.
By the start of the war in Europe, Germany had the largest submarine fleet of all the major powers. And for the first few years of the war, they had much of the Atlantic Ocean in their grip. While merchant ships tried to bring supplies to Britain, U-boats gathered in “wolfpacks” to attack – assigning multiple submarines to hit a single target.
Things changed when the United States joined the war. With an increased Allied presence in the Atlantic, the flaws in the U-boats began to show. Essentially, they were just ships with the capacity to dive – they were slow and ungainly underwater and they required extended amounts of time to be spent on the surface.