Cabras Island was historically a low-lying finger of land off the coast of Piti, Guam that formed part of the northern protective arm of Apra Harbor.

Shortly after the 1944 Battle of Guam it was connected by a causeway to the mainland and extended by the Glass Breakwater, and is now typically referred to simply as Cabras.

Before the twentieth century, Apra Harbor was protected by the Guam mainland to the east; Cabras Island, Luminao Reef, and Calalan Bank to the north; and Orote Peninsula to the south.

Among the improvements recommended by a 1930s board headed by Rear Admiral Arthur Japy Hepburn in the run-up to World War II was a breakwater upon Luminao Reef to the west of Cabras Island.

The northern arm of Apra Harbor, from the main island through Cabras to the tip of the breakwater at Spanish Rocks measured 17,000 feet (5,200 m).