Final answer:
Germany's invasion of neutral Belgium and unrestricted submarine warfare, including the sinking of ships with American citizens, were critical factors in the U.S. declaring war on Germany and joining the Allied Powers in World War I.
Step-by-step explanation:
The United States saw Germany as an enemy primarily due to Germany's aggressive military strategies and violations of neutral rights. One such event was Germany's invasion of Belgium, a neutral country, which was part of its plan to quickly attack France. This act not only breached Belgium's neutrality but also incited Great Britain to join the war due to its treaty with Belgium. Additionally, Germany's submarine warfare, which targeted and sank ships regardless of neutral status, including those with American citizens, profoundly affected U.S. sentiment. The most notable sinking was that of the Lusitania, which carried American passengers and stoked public opinion against Germany. These actions, combined with other strategic concerns such as the Zimmerman Telegram, compelled the United States to abandon its initial isolationist stance and declare war on Germany in April 1917, joining the Allies in World War I.