Why did germany's use of U-boats lead to conflict with the US?

Answer 1

In World War I, Germany's use of U-boats (or, submarines) led to conflict with the United States for two reasons. First, the general American reaction to the use of U-boats, especially when utilized in an unrestricted manner, was thoroughly negative. This kind of warfare was often considered to be unlawful, cruel, even unfair. Second, the loss of American lives and goods when the U-boats began to sink American ships (despite their neutrality) led to war-fever at all levels of American society.

Answer 2

It wasn't the use of boats, but "U-boats" that led to the conflict. "U-boat" is from Unterseeboot, which is German for "under-sea-boat", or submarine.

During World War I, the powerful British Navy set up a blockade to prevent countries from trading with Germany. Since Germany's navy wasn't powerful enough to stop it, Germany instead began using their U-boats to sink enemy ships. However, this was not able to stop the blockade, so the Germans had to resort to more ruthless strategy.

The Germans began what is commonly called "unrestricted submarine warfare". This meant the U-boats would attack any ships, even ships from countries that weren't actually fighting in the war, such as the US. Additionally, the U-boats would attack without warning- normally a submarine would issue a warning and give the crew and passengers a short time to get off the ship before the submarine actually sank it. The Germans stopped giving the warning and would just attack the ship, meaning civilians were more likely to be killed.

In 1915, after countries like the US got upset at the unrestricted submarine warfare, the Germans agreed to stop and go back to the normal rules. The British blockade remained, and as the war dragged on, the Germans got desperate and needed to break the blockade. In spring of 1917, the Germans decided they would restart the unrestricted warfare- and since they knew the US would be upset about it, they also sent a message to Mexico, asking them to start a war with the US, to try to keep the US from joining World War I. British spies intercepted the message and told the US about it. So this message, along with the German renewal of unrestricted submarine warfare, sufficiently angered the US that war was declared in April 1917.