U-boats: Definition & Facts

Instructor: Flint Johnson

Flint has tutored mathematics through precalculus, science, and English and has taught college history. He has a Ph.D. from the University of Glasgow

Learn about U-boats, the submarines used by the German navy to interrupt shipping lanes to the British Isles during World War I and World War II. They were the only German threat Churchill ever worried about.


U-boat is just the English version of Unterseeboot or undersea boat. The word U-boat was made famous during the two world wars. During those wars, the Germans used U-boats to attack all ships going to the British Isles.

You see, for hundreds of years, the British had controlled the seas throughout the world because their navy was the largest, the best armed, and the most disciplined in the world. The Germans, who had no real naval tradition, realized that they could not compete with Britain's battleships, so instead they developed boats that could fight using stealth. From 1850 on, the Germans developed submarine designs for the sole purpose of helping Germany neutralize the British Navy.

A U-boat model U-9 from World War I

World War I

As soon as war broke out, it was clear that the Germans had a good weapon in the U-boat. U-boats were able to sink British ships quietly and safely. It was soon learned, though, that the U-boat could be more effective by sinking commerce vessels.

Of course there were rules to war, and one of them was that anyone without a gun (noncombatant) was to be protected whenever possible. That meant that a commerce ship had to be given fair warning before it was to be sunk so that its passengers could be given the chance to escape. That worked well for the Germans, who sank ships at will.

So the British began disguising their destroyers like commerce vessels, and the next time a German captain announced he was going to sink a ship, he was gunned down and his U-boat sank. Naturally, the Germans stopped warning the ships they wanted to sink.

The Bigger Picture

The British were on an island, and they received goods from all over the world. By sinking any vessels going to the islands, the Germans were attempting to keep food and supplies from getting to Britain. If the U-boats were going to be useful, they had to sink any ship going to Britain, including ships from neutral countries. When they had warned ships before sinking them it had irritated the neutral countries. Once they began sinking without warning, they were killing citizens of neutral countries.

The Germans tried to find some balance between hurting the British and protecting neutral countries, but were unable to. They declared all ships going to Britain to be targets in February of 1917. In May of 1917 a U-boat sank the Lusitania, and the U.S. joined the war. The U.S.'s navy, coupled with the introduction of radar and convoys protected by destroyers, would make the U-boats almost useless by the end of the war.

2 U-boats from World War II

World War II

The Treaty of Versailles, which had ended World War I, had severely restricted the German navy, but the Germans had been able to construct U-boats and put them into the Atlantic immediately when World War II started. They caused a lot of damage.

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