What did Germany do in 1915 that tested the United States neutral stance in World war 1

In February 1915, Germany announced unrestricted warfare against all ships, neutral or otherwise, that entered the war zone around Britain. One month later, Germany announced that a German cruiser had sunk the William P. Frye, a private American vessel that was transporting grain to England when it disappeared.

America was a close trading partner of Britain during world war one and this partnership has lead to arose tension between America and Germany, Several American ships were sunk by the German navy.

In February 1915, the German government has declared unrestricted warfare against all the ships remain neutral or otherwise they have to face the German navy.

After this declaration German navy sunk down a private American vessel that was supplying food items to Britain, When Wilson administration outraged about this incident German government apologize that they did this by mistake.