At first assigned to surface ships, the young seaman switched to U-boat service in November 1915, by which time World War I had begun.

By 1918, the Kaiser's fleet of U-boats had acquired sufficient strength to be able to sight, and sink, a substantial percentage of the ships that attempted to deliver necessary supplies to Great Britain.

Germany had declared unrestricted submarine warfare against Great Britain, which in its eyes gave it the right to "sink on sight" ships bound to or from the enemy nation.

In compliance with the laws of war accepted by all of the European combatant nations – including Germany – at that time, the Llandovery Castle had been marked with painted and lighted Red Crosses to signify its status as a noncombatant vessel.

The indictment against Patzig was quashed in absentia in 1931 as an acknowledgment by the German courts of the enactment, by the Reichstag, of two laws of amnesty that applied to his case.

[citation needed] In World War II Patzig, now under the name Brümmer-Patzig,[2] again saw service in the German navy's U-boat arm.

[3] From November 1941 until March 1943, he served as a torpedo attack instructor in the 25th U-boat Flotilla, a training group based in the Baltic Sea.