When Martínez was named Commissioner, Sisto quickly staged a coup d'état and claimed the position as the highest ranking Spanish official on the island.

He began arming native guards and commandeering ammunition, but was briefly overthrown by Venancio Roberto and other pro-American elements on December 31, 1898, but was officially put into power by officers of the United States Navy only two days later after they decided he held a legitimate claim to the position.

After the Capture of Guam by the United States during the Spanish–American War, the territorial Spanish government was deposed; eventually, Francisco Portusach Martínez, the only American citizen on the island, was named Commissioner.

[4] Eventually, local district leaders, including Father José Palomo and former governor Martínez removed Sisto from office and placed Venancio Roberto in the position on December 31, 1898.

[1] When news of the 1898 Treaty of Paris reached the island, Sisto acknowledged that ownership of Guam had officially been transferred to the United States, and relinquished his position on February 1, 1899.