John Harris became the new Commandant of the Corps after the death of Archibald Henderson. Harris was born in Pennsylvania and joined the Corps during the War of 1812. In the War with Mexico, he took a battallion to the field but was too late, the war had ended before he got there with his troops. He never had a flashy career as an officer but was also solid and did his job.
Harris started the job just before the start of the Civil War and was accused of having Southern sympathies. When the states started to leave the Union, over half of the officers of the Marine Corps left. In fact the story goes that Harris gave one of the officers a letter of recommendation. During the war, he made no effort to expand the scope of the Corps and often came into conflict with Gideon Welles, who was Secretary of the Navy. He died on May 12, 1864 and a successor was chosen.
Information from this node was taken from my bookshelf and the offical Marine Corps history page. Note: www.marinecorps.com is not the offical page. http://hqinet001.hqmc.usmc.mil/HD/Historical/Frequently_Requested/Commandants.htm