Unrestricted Submarine Warfare: Definition & Concept

Instructor: Christopher Prokes

Chris is an instructional designer and college faculty member. He has a Master's Degree in Education and also umpires baseball.

Germany used unrestricted submarine warfare in World War I effectively both offensively and defensively against its enemies. Learn about this vital military strategy in this lesson and take a short quiz at the end.

Dive, Dive, Dive!

Have you ever seen a submarine, perhaps in a museum? They operate sight unseen underwater and have many capabilities for battle. This vessel became well known in World War I. Germany, an aggressor, followed a policy of unrestricted submarine warfare by attacking any and all vessels that sailed through certain waters. In this lesson, learn how this approach worked, why Germany viewed it as vital both offensively and defensively, and its legacy and impact.

How it Worked

Like a shark lurking to prey, a submarine can sneak up on a ship and sink or destroy it quite easily. This advantage was especially potent in World War I. The German Navy used the Unterseeboot or U-boat to terrorize vessels on the seas.

Example of an Unterseeboot, or U-boat

There were two main reasons Germany sought to use U-boats. Great Britain (already involved in the war) tried to cut off and choke Germany by enacting a blockade around it. U-boats could break the blockade. Secondly, the United States (not yet involved) had an indifferent attitude towards the blockade and was a supply ally to Britain.

In response, Germany began an approach of sinking any and every vessel that it encountered either in its own waters or near Great Britain. And they meant every boat; no discrimination was made between passenger and military. The German Navy realized sinking ships was far more meaningful than capturing them. Enter the term unrestricted submarine warfare. One of the most famous examples was the Lusitania, a British passenger liner, where nearly 1,200 people were lost (including many Americans).

The Lusitania was the most famous passenger ship sunk

Defense or Offense?

Germany would suspend their use of unrestricted submarine warfare, though briefly, from late 1916 until February 1917, worried that the United States would join the war and turn the tide. More on that later. Regardless of when and where they used this approach, the key issue is to consider whether unrestricted submarine warfare was a strategy of aggression (offense) or prevention (defense). It may help to think of football strategy as there are two sides with many plays, schemes, and approaches.

On the defensive end, there were three main plays in the German playbook. First, there were attempts at peace early on in WWI. Many of these attempts sought to severely harm Germany in the aftermath of war, and there was no way German leaders would accept any such terms. Thus, submarine attacks were necessary to flex their muscles.

A second scheme dealt with rules of engagement for naval battles. It was customary for a ship to warn another of engagement, such as by a warning shot. The Germans argued that submarines, due to the fact their hulls could be easily breached by weapons, could not always warn ships they encountered before attacking. Defense by destruction was the only method of engagement, as no time could be spent to figure out if the ship was friendly. Similarly, Germany had a smaller navy and had to be aggressive when dealing with the British Navy, then strongest in the world.

A final defensive play dealt with the blockade around Germany, though some related offensive reasons did exist (see below). Germany protested the blockade because at the time, international maritime policies (including blockades) were vague and open to interpretation. They were upset that the United States had a cozy relationship with Britain and didn't do much to oppose the supposedly unacceptable blockade. Germany had to then defend itself as others wouldn't help them.

Geographic area of unrestricted submarine warfare (England is located on the larger island in the middle)
Submarine Zone

On the other side of the ball, there were three main offensive plays. As noted above, the blockade was in place and opposed by Germany. Its people were starving and the blockade prevented supplies from reaching those in need. Also in need, of course, were military units, and neither could afford not to be supplied by their allies due to the blockade. Destruction of other ships around Britain was necessary to end the restrictions--kind of an eye-for-an-eye approach, if you will.

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