The action of 15 October 1917 was a naval engagement of World War I between Imperial Germany and the United States off the coast of Mine Head, Ireland.

The German submarine U-61—on a typical unrestricted U-boat mission — was cruising in British waters, attacking Allied shipping.

The American destroyer began to steam in circles, but returned a barrage of 4 inch shells which forced the U-boat to dive.

Eventually, another American destroyer USS Porter and the British sloops HMS Jessamine and Tamarisk arrived on the scene and protected Cassin throughout the night.

The damaged USS Cassin was repaired and returned to active duty in July 1918; U-61 was sunk by the P-class sloop HMS PC.51 a few months later.