On April 4, 1776, after the American Revolutionary war had been raging in Massachusetts for nearly a year, the Provincial Congress at Halifax chose Sumner to be colonel, and thus commanding officer, of the 3rd North Carolina Regiment. He likely participated in the defense of Charleston against a British invasion attempt in 1776, after which he was involved in the aborted plans of Major General Charles Lee to invade British Florida. During the planning stages for the Florida invasion, Sumner disagreed with Peter Muhlenberg of the 8th Virginia Regiment over which of the two was to be given command over Lee's Virginia and North Carolina troops while the commanding general was traveling in advance of his men. This dispute was resolved only when a military court of inquiry awarded Muhlenberg temporary command after Sumner failed to appear and plead his case. By August 18, 1776, Sumner's 3rd Regiment had reached Savannah, Georgia, where they joined Lee, who had arrived earlier in the month. The planned invasion of Florida did not materialize, though, and Sumner left his regiment at Savannah in September 1776 to recruit more soldiers from North Carolina.