In the late 1840’s Americans imported guano (seabird dung) as fertilizer. The best islands for this were controlled by South American countries. In 1856 William Seward introduced a bill to allow the US to take possession of guano islands outside foreign nations for our American farmers. The bill did not require the US to give up the islands after they were no longer needed. The US then claimed islands including Johnson and Midway which ended up being important bases in WWII. Today there are still dozens of islands owned by the US because of this bill.
(Book – Seward: Lincolns Indispensable Man – Walter Stahr)