Part of the nation's Third System of Coastal Defense, Fort Massachusetts was built to protect a deep water pass on the sea approach to New Orleans. Started in 1859, its construction by the U.S. Army Corp of Engineers was hindered by the harsh environment, hurricanes, yellow fever epidemics, and the Civil War. Local Confederate militia held the island and the uncompleted fort for 9 months in 1861. A brief naval battle occurred in July 1861 between the Confederate forces on Ship Island and the sailors on-board the USS Massachusetts. The Union forces took over the island in September 1861 after the Confederate forces vacated. Construction of the fort resumed during the remainder of the Civil War and was completed in 1866. Fort Massachusetts still stands, but all of Ship Island's other buildings from the 1800s disappeared along with the military. During the Civil War, the population of the U.S. Army and Navy ranged from fewer than 100 up to at least 20,000. The island acted as a bustling small city, with soldiers and sailors unloading supply ships, cleaning stables, guarding prisoners, cooking meals, tending the sick and digging wells in the sand. Federal military convicts, political dissidents, and Confederate soldiers were also imprisoned on Ship Island during the Civil War. Civilian detainees arrived in June 1862 after the Union capture of New Orleans. The first of nearly 5,000 Confederate prisoners of war arrived in 1865. Over 3,000 came after the battle of Mobile Bay in 1865. Diseases killed 153 Confederates and over 260 Federal soldiers. The prisoners were guarded by the 2nd Regiment of the Louisiana Native Guards who arrived in 1863. At first these African-American soldiers erected batteries and guarded military convicts. In April 1863, the regiment clashed with Confederate troops at East Pascagoula--the first black unit engaged in combat on the Gulf Coast. Several black companies remained on Ship Island until 1870, when they were assimilated into the 25th Infantry and became known on the Texas frontier as the Buffalo Soldiers.