[138] In December 1817, Secretary of War John C. Calhoun initiated what would be called the First Seminole War by ordering Jackson to lead a campaign "with full power to conduct the war as he may think best".[139] Jackson believed the best way to do this was to seize Florida from Spain once and for all. Before departing, Jackson wrote to President James Monroe, "Let it be signified to me through any channel ... that the possession of the Floridas would be desirable to the United States, and in sixty days it will be accomplished."[140]
Jackson invaded Florida, captured the Spanish fort of St. Marks, and occupied Pensacola. Seminole and Spanish resistance was effectively ended by May 1818. He also captured two British agents, Robert Ambrister and Alexander Arbuthnot, who had been working with the Seminoles. After a brief trial, Jackson executed both of them, causing a diplomatic incident with the British. Jackson's actions polarized Monroe's cabinet. The occupied territories were returned to Spain.[141] Calhoun wanted him censured for violating the Constitution, since the United States had not declared war on Spain. 