Final answer:
The sinking of the Lusitania by German U-boats led to a shift in American public opinion and incremental movement towards war. While initially trying to de-escalate with the Sussex Pledge, Germany's continuation of aggressive submarine warfare eventually pushed the US to declare war in April 1917.
Step-by-step explanation:
The United States responded to the sinking of the Lusitania by German U-boats with increasing tension and a move towards military preparedness. Initially, the American public was outraged by what they saw as an unprovoked attack, and Germany defended its actions as a legitimate form of self-defense. Despite warnings from the Germans about traveling on British vessels, the loss of American lives led to a shift in public sentiment. In the aftermath, Germany attempted to alleviate tensions by promising not to sink more civilian vessels without ensuring the safety of passengers (known as the Sussex Pledge), but the promise proved impractical in wartime conditions.
Before America's official entry into World War I, there were several responses to the aggressive submarine warfare conducted by Germany, including Congress approving the National Defense Act of 1916, which expanded the US military and authorized the construction of new ships. Despite these actions, it was not until the continued sinking of American ships and loss of American lives, including the attacks on the American merchant ship Laconia and subsequent vessels, that President Wilson was pressured into declaring war on Germany in April 1917.
This shift towards an offensive stance was solidified by the intensification of German unrestricted submarine warfare in 1917, which included the sinking of multiple American ships, driving the United States closer to war. The sinking of the Lusitania and other aggressive U-boat actions contributed to the US joining the Allies in World War I.