1. During World War I (1914-1918):
- Lusitania Sinking: In 1915, a German U-boat (submarine) sank the British passenger ship Lusitania off the coast of Ireland, killing over 1,200 people, including 128 Americans. Public outrage in the US grew due to the loss of American lives, bringing the country closer to war with Germany.
- Unrestricted Submarine Warfare: Germany's adoption of unrestricted submarine warfare in 1917, meaning they targeted any passing ships (including neutrals) in an attempt to cut off British supplies, further heightened tensions with the US.
After these incidents, the United States eventually declared war on Germany in April 1917, joining the Allied Powers and playing a significant role in turning the tide of the war.
2. World War II (1939-1945):
- German U-boat Attacks: As the war escalated in Europe, Germany's submarine attacks and naval blockades in the Atlantic posed a threat to both British shipping and US interests there. The sinking of American merchant vessels and naval ships by German U-boats brought US-Germany relations to a breaking point.
- Isolationist Policies: Initially, the US maintained an isolationist stance and remained reluctant to enter the war directly. However, President Franklin D. Roosevelt took steps to support Great Britain with various trade agreements and aid programs. Germany's invasion of Poland in September 1939 prompted the US to increase its support for the Allies.
Despite these tensions, the United States formally entered World War II in December 1941 after the surprise attack on Pearl Harbor by Imperial Japan, an ally of Germany and Italy.
In both World War I and World War II, the events described led to critical decision-making moments for the United States as the country faced the choice of declaring war and getting directly involved in these major international conflicts.