The fighting in World War I was ended by what Wikipedia refers to as the Armistice of 11 November 1918. The Treaty of Versailles was signed on 28 June 1919; in the words (again) of Wikipedia, it "ended the state of war between Germany and the Allied Powers."
Had the question referred to 1919, we'd have let it pass. But as it stands, "the armistice" is a better answer.
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