ORLEANS (AP) — A Cape Cod town is celebrating the 100th anniversary of the only enemy attack on American soil during World War I.

A German U-156 submarine shelled a tugboat near Orleans, Massachusetts, on July 21, 1918, the first attack on the U.S. in 100 years.

A commemoration is taking place Saturday evening on Nauset Beach to mark the anniversary.

It remains a mystery why an advanced submarine would attack a target that had no real value. One theory is that the sub had hoped to cut the underwater communications cable that ran from Orleans to France. The U-boat sent hits to the tug boat Perth Amboy and its four barges, and left an hour later.

The submarine attacked other ships near Canada, but disappeared in September 1918.