From SFWiki

Treasure Island is an artificial island build as a WPA project for the 1939 Golden Gate International Exposition by dredging dirt from the bay. It is connected by a land bridge to the naturally formed neighbor Yerba Buena Island.

The island was supposed to become an airport after the exposition closed in 1940, but the plan was derailed by World War II. Then Mayor Angelo Rossi reached an agreement with the US Navy allowing them use of the island “for the duration of the war and for five years thereafter” in the war effort.

Less than a month later the island was seized by the US Navy, offering only $44,801 to cover the $3,691,000 the city had invested in improvements to the island.