Other than germany's use of unrestricted submarine warfare, what other incident angered americans and catalyze american entry into world war i?

The sinking of the passenger liner RMS Lusitania in 1915 by a German submarine was the first incident that made the United States seeing Germany as an aggressor. The hostilities continued for some time due to Germany’s use of unrestricted submarine warfare. However, what catalyze American entry in World War I was Germany’s secret offer to Mexico. Germany proposed Mexico to become its ally, by offering it help to recover lost territories in the Mexican-American War. The message was sent by telegram, known as the Zimmermann Telegram, but it was intercepted by British intelligence. After this, the Congress decided to declare war on Germany on April 6, 1917 and declared war on Austria-Hungary on December 7, 1917.

Background/detail:

In 1915, public outrage in the US against the Germans swept the nation following the sinking of the British ocean liner, Lusitania.    When a German U-boat (submarine) sank the Lusitania in May, 1915, over 1,000 persons were killed, including more than 100 Americans. The passenger liner was targeted by the Germans because they suspected weapons were being shipped to Britain in the cargo hold of the ship.

Germany managed to stave off American entry into the war at the time by pledging to stop submarine attacks.

In January, 1917, Germany resumed its policy of unrestricted submarine warfare.  Germany had halted its attacks on non-military vessels (which it suspected of carrying military supplies) after the furor over the sinking of the Lusitania in 1915.  But in 1917, Germany resumed attacks by its U-boats.

In February, 1917, the "Zimmermann Telegram" was intercepted by British intelligence and shared with the US.  Germany's foreign minister, Arthur Zimmerman, had telegraphed an offer to Mexico's ambassador seeking Mexico's support in war vs. the United States in exchange for getting land back from the US.

On April 2, 1917, President Wilson made a powerful speech to Congress in which he argued that the nation needed to enter the war "to make the world safe for democracy."  Wilson's speech was powerfully convincing, and four days later, Congress declared war.

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