Why did german submarine warfare upset the united states?

U-boats were the reason for the sinking of the Lusitania, which was carrying civilians and artilley

German submarine warfare upset the United States because they made advancements that we had not had yet with our own technological warfare. They had created a device that could go undetected on a radar, and the United States wanted to turn the tables, so they tried to one up the Germans by trying to make an entire warship invisible, not only to the radar, but to the naked eye. As it turns out, they accidentally made the entire ship disappear, and when it returned, the entire crew was either dead, or melded into the metal on the ship. Like, they were a part of the ship...literally melted into it. It was called the Philadelphia Experiment.