Most submarines were combat submarines In contrast with a blockade runner and merchant submarine such as Deutschland.

Submarines were critical to Germany's war effort, and were needed to offset the superiority of the British Royal Navy to the Imperial German Navy.

Germany's sinking of three British battleships on September 22, 1914, drove the British from the North Sea for a time. The imposition of the British blockade of Germany led to Germany's declaration of a war zone around the British Isles and the unrestricted submarine warfare. Germany never had sufficient submarines to both protect its surface fleet and conduct unrestricted submarine warfare.

The major powers had submarines.

British submarines operated in the Baltic Sea and in the Dardanelles and Sea of Marmora.

French submarines operated in the the Mediterranean Sea and in the Dardanelles.

Austro-Hungarian submarines operated in the Mediterranean Sea from its home port of Pola in the Adriatic Sea.

Combat Submarine is a type of Submarine.