Question

After its U-boats sank the Lusitania in 1915, what did Germany promise to do?

Answer 1

After its U-boats sank the Lusitania in 1915, Germany promised to "limit submarine warfare", since they wanted to avoid a full-on conflict with the United States. 

Answer 2

The Lusitania carried 1,128 people including 128 Americans, so this strained relations among the United States and Germany. Germany wanted to avoid the U.S. entering the War so even when they argued that the passenger ship was carrying weapons and supplies from the U.S. to Europe they promised that they will no longer sink passenger ships without a warning. That made President Wilson chose not to declare war on Germany at that time.