What was the significance of the Zimmerman note?

The Zimmerman note, or telegram, was significant because it was one factor leading to American involvement in World War I. The German navy had used submarine warfare to disrupt British maritime trade, but this nearly caused a conflict with the United States, which protested when ships bearing American passengers were torpedoed. Eager to avoid war with the United States, the German imperial government pledged in 1916 not to attack American merchant or passenger ships without warning. But they decided in 1917 to renege on this pledge and resume submarine warfare without restrictions. This angered the US government under Woodrow Wilson, which nevertheless still did not declare war on Germany. The "note" in question was a telegram sent from the German foreign minister, Arthur Zimmerman, to the country's diplomatic representative in Mexico. It outlined the following plan: If the United States declared war on Germany, then Mexico should declare war on the United States, and Germany would ensure, at war's end, that Mexico received the lands that it had lost as part of the Treaty of Guadelupe Hidalgo in 1848. British intelligence intercepted and translated the coded telegram, however, and when it was made public, it played a major role in turning American public opinion against Germany and for war. Just one month after the contents of the telegram were published in American newspapers, the United States Congress declared war on Germany in April 1917.