What allowed the united states to claim american samoa?

It is the Guano Islands Act approved last August 18, 1856. It empowers residents of the United States to take ownership of unclaimed islands containing pertaining guano deposits as long as it is not occupied and not within the reach of the power of another government. It also authorizes the President to take the military to guard and protect all the established criminal authority of the country.

Treaty and negotiations with Germany

Initial treaty in the 1870s and a negotiation with Germany to create America Samoa.

The US created a land treaty with the Samoan government in the 1870s to secure land for navy ports. This treaty created a US interest in the Samoan Islands. Disputes over the islands began in 1898 with a civil between the Samoan people where the US took one side and the German another. In 1899, to prevent war between Germany and the US, a negotiation was agreed upon to split the islands with the west going under Germany control and the eastern islands going under the control the US to become America Samoa.