In ten days of recruiting, however, Palmer received enough applications for enlistment to form a regiment, which was authorized as the 15th Pennsylvania Cavalry. He was appointed the regiment's colonel. Before Palmer was able to organize the regiment at Camp Alabama in Carlisle, Pennsylvania, he and a portion of it were ordered on September 9 to help the Army of the Potomac resist the Confederates invasion of Maryland. For nearly a week Palmer, accompanied by a telegrapher, personally sought information of Lee's movements every night in civilian clothing, and transmitted his findings to General George B. McClellan via telegraph connections.