Soulé contributed to the "tragicomic character of American diplomacy"[3] in Europe during this period in that "He wounded the French ambassador in a duel, handed down an unauthorized ultimatum, and tried to pick a war with Spain over the Black Warrior affair."[4] During this period, Soulé became known for writing the 1854 Ostend Manifesto, part of an attempt by Southern slaveholders of the planter class to gain support to annex Cuba to the United States.Worried about being bounded by free states to the north and west, some prominent Southerners wanted to expand their territory to the Caribbean and into Central America.[5] In late 1852, while in Washington, D.C., Soulé had provided some support and assistance to the agent responsible for rescuing Solomon Northup, a free black from Saratoga Springs, New York, who had been kidnapped and sold into slavery.In 1861, he supported organizing the Allen Rifles which was Company 1 of the 26th Louisiana Infantry Regiment and gave an impassioned speech at a big barbecue in Thibodaux in Lafourche Parish.[7] On May 18, 1861, Soulé was captured by federal troops, charged with "plotting treason against the United States government," and imprisoned in Fort Warren, Massachusetts.