Why did the us protest against the submarine warfare that Germany used?

The United States protested against Germany's submarine warfare primarily because it violated international laws regarding unrestricted warfare and the rights of neutral nations. Germany's use of U-boats to target merchant and passenger ships, including those carrying American citizens, posed a direct threat to U.S. lives and commerce. The sinking of ships like the Lusitania in 1915 heightened tensions, leading to calls for the U.S. to take a stand against Germany's aggressive tactics. Ultimately, these actions contributed to the U.S. decision to enter World War I in 1917.