Final answer:
Several key events needed to happen for the United States to become involved in World War I, including the Zimmermann telegram, President Wilson's mobilization efforts, and American troops providing reinforcements.
Step-by-step explanation:
In order for the United States to become involved in World War I, several key events had to occur.
First, the United States needed a reason to enter the war. This came in the form of the Zimmermann telegram, where Germany urged Mexico to declare war on the United States. This threat to American security led to a shift in public opinion and a call for action.
Second, President Woodrow Wilson needed to mobilize the American people and resources for war. He passed legislation and encouraged citizens to support the war effort through initiatives like buying liberty loans. This ensured that the United States had the necessary manpower and materials to fight in Europe.
Finally, the United States needed a decisive moment on the battlefield. When American troops arrived in France, they provided much-needed reinforcements to the weary Allied forces and helped turn the tide of the war in their favor.