A native of Pennsylvania, Adam J. Slemmer was graduated from West Point in 1850 as brevet second lieutenant 1st Artillery. He was promoted to first lieutenant in 1854. Slemmer was in charge of the small artillery garrison quartered at Fort Barrancas, near Fort Pickens, when the secession crisis occurred. In early January 1861, Slemmer considered his situation at Fort Barrancas to be untenable. Florida's secession was imminent, certain federal posts in Florida and Alabama had already been seized, and hostile troops were gathering at Pensacola. Deciding to concentrate his forces to defend only one fort, Slemmer moved his troops on January 10 to the greater security of Fort Pickens, then unoccupied. Two days later, Florida and Alabama troops took the mainland bases and demanded that Slemmer surrender Fort Pickens. He refused these demands and held his position until an informal agreement, or "truce," was established between the Buchanan administration and Florida. Southern troops would not attack Pickens so long as Union troops remained aboard nearby ships and did not reinforce the fort.