How did the u boat campaign almost win world war 1?

Answer 1

It wasn't won using U-boats, but it was one of Germany's most effective strategies. The Battle of the Atlantic was fought by U-boats against the convoys taking supplies across the ocean. If they had been able to prevent the convoys getting through, the Germans might have been able successfully destroy England and Russia.

Answer 2

Great Britain relied heavily, even in peace time, on the hundreds of cargo ships travelling every day to and from her ports and to and from the various colonies that made up her empire. Any interruption of this sea-going trade endangered her very survival. If the German U-boats could sink enough of those cargo ships, England would not be able to stay in the fight. While the U-boats were effective and hurt Great Britain severly, there were not enough of them to be decisive. Even if there had been enough, the British adapted quickly to the threat employing better and better technology.