On May 7, 1915, a German U-boat torpedoed the British-owned luxury steamship Lusitania, killing 1,195 people including 128 Americans, according to the Library of Congress. The disaster immediately strained relations between Germany and the neutral United States, fueled anti-German sentiment and set off a chain of events that eventually led to the United States entering World War I.

Lusitania, owned by the Cunard Shipping Line, was launched in 1906 to carry passengers on transatlantic voyages. The British Admiralty subsidized the ship’s construction with the understanding it would be pressed into military service if war broke out. After World War I began in 1914, Lusitania remained a passenger ship, although it was secretly modified for war.

By February 1915, German naval commanders knew British merchants were arming their ships and that both merchant and passenger ships were transporting weapons and supplies from the United States to Europe.

As a result, Germany declared the waters surrounding the British Isles a war zone and stopped following international naval “prize laws,” which warned ships of a submarine’s presence. This break from naval protocol angered and troubled the United States and the European Allies.

Days before Lusitania was scheduled to leave New York for Liverpool in early May 1915, the Imperial German Embassy in Washington D.C. placed ads in American newspapers reminding Americans that Britain and Germany were at war. They warned potential travelers that “vessels flying the flag of Great Britain or of any of her allies are liable to destruction” and should be avoided.

Since it was assumed Germany would still allow passengers to get into lifeboats prior to an attack, the cautions were largely ignored.

On May 7, 1915, six days after leaving New York for Liverpool, Lusitania took a direct hit from a German U-boat submarine—without any warning—and sank within 20 minutes.

As word spread about Lusitania’s tragic fate, so did the outrage. American citizens were saddened and stunned but not ready to rush to war. President Woodrow Wilson wanted to proceed with caution and remain neutral while former President Theodore Roosevelt demanded swift retaliation.

Germany defended its aggression, claiming Lusitania had carried weapons and war supplies and was therefore fair game. As they continued to divert blame, British propaganda against them snowballed. Throngs of vengeance-seeking Brits rushed to enlist, and anti-German riots broke out in London.

Said Winston Churchill, First Lord of the Admiralty, “The poor babies who perished in the ocean struck a blow at German power more deadly than could have been achieved by the sacrifice of 100,000 men.”

In August 1915, a German submarine sunk the British ocean liner S.S. Arabic and claimed self-defense. The event further strained diplomatic relations between the United States and Germany. President Wilson warned Germany that if it was determined they’d sunk the ship without cause, the United States may cut diplomatic ties and enter the war.

Germany caved, and in September announced they’d no longer sink passenger ships without warning. Satisfied, at least for the moment, President Wilson chose not to declare war on Germany despite being encouraged otherwise by some of his cabinet members.

The sinking of Lusitania was a public relations nightmare for Germany as public opinion in the United States turned against them. But President Wilson still wasn’t ready to take his country to war.

Then, in early 1917, Britain intelligence intercepted a telegram from German Foreign Minister Arthur Zimmerman to the German Minister to Mexico Henrich von Eckhardt.

The Zimmerman telegram stated that Germany planned to return to unrestricted submarine warfare and would sink all ships—including those carrying American passengers—located in the war zone. The telegram also proposed an alliance between Germany and Mexico should the United States decide to join the European Allies.

President Wilson was outraged but still didn’t enter the war. However, when Germany officially resumed unrestricted submarine warfare, Wilson and the American public had had enough. In April 1917, the United States Congress voted to declare war on the Central Powers and entered World War I.

The sinking of Lusitania didn’t directly cause the United States to enter the war. It did, however, fuel virulent anti-German sentiment in Britain and the United States and hinder diplomatic relations between Germany and the United States.

It also showed the world that Germany was willing to do almost anything to win the war, which incited the Allies to fight harder and signaled to the United States that permanent neutrality was likely futile.