The American government reacted to the sinking of the Lusitania by trying to stop Germany from using unrestricted submarine warfare. They did not want to join World War I, they wanted to try to remain neutral.
President Woodrow Wilson tried his best to keep the United States out of the war. He felt that joining World War I would not be best for the country or for the people, so despite the Lusitania sinking and Germany continuing to use unrestricted submarine warfare, he kept the United States neutral for a while. Many American people were outraged by the sinking of the Lusitania and wanted to join the war, but the government continued to stay out of it.
This neutrality turned out to not be long-lasting, as the British had intercepted a telegram that was going from Germany to Mexico. This telegram, known as the Zimmermann Telegram, stated that if Mexico were to attack the United States, they would be given land when Germany won the war. Between the Lusitania, Zimmermann Telegram, and Germany continuing to use unrestricted submarine warfare, despite them saying numerous times they weren't, President Woodrow Wilson saw this as the final straw. He asked Congress to declare war on Germany and the United States had finally joined World War I.