Final answer:
The United States maintained a neutral, isolationist stance at the beginning of World War I but declared war on Germany in 1917 after a series of incidents, including unrestricted submarine warfare that led to American civilian casualties.
Step-by-step explanation:
When World War I broke out in 1914, the United States, under President Woodrow Wilson, maintained a stance of neutrality, taking no active part in the hostilities. The U.S. adopted isolationism, aiming to avoid becoming entangled in overseas conflicts despite escalating tensions globally.
As the war progressed, and after numerous provocations, including attacks on ships carrying American citizens, the United States altered its position and joined the Allies in 1917. The immediate cause for this shift was Germany's unrestricted submarine warfare, which saw the sinking of civilian and merchant ships, including the notable Lusitania which carried American passengers. These events, coupled with other factors, such as the Zimmermann Telegram, led to public sentiment turning against Germany. On April 2, 1917, President Wilson asked Congress to declare war on Germany, stating that the world must be made safe for democracy, marking the official entry of the United States into World War I.