Given the question: Information:  - The Passaic - class ironclad monitors of the U.S. Navy saw service in the U.S. Civil War and the Spanish -- American War . The last such monitor was stricken from the Navy List in 1937 . This highly successful class was an improved version of the Monitor equipped with a XV - inch Dahlgren gun in place of one of the XI - inchers .  - The American Civil War was a civil war in the United States fought from 1861 to 1865. The Union faced secessionists in eleven Southern states grouped together as the Confederate States of America. The Union won the war, which remains the bloodiest in U.S. history.  - John Adolphus Bernard Dahlgren (November 13, 1809  July 12, 1870) was a United States Navy officer who founded his service's Ordnance Department and launched major advances in gunnery.  - Dahlgren guns were muzzle-loading naval artillery designed by Rear Admiral John A. Dahlgren USN (November 13, 1809  July 12, 1870), mostly used in the period of the American Civil War. Dahlgren's design philosophy evolved from an accidental explosion in 1849 of a 32-pounder being tested for accuracy, killing a gunner. He believed a safer, more powerful naval cannon could be designed using more scientific design criteria. Dahlgren guns were designed with a smooth curved shape, equalizing strain and concentrating more weight of metal in the gun breech where the greatest pressure of expanding propellant gases needed to be met to keep the gun from bursting. Because of their rounded contours, Dahlgren guns were nicknamed "soda bottles", a shape which became their most identifiable characteristic.    After reading the paragraphs above, choose the best answer for the entity that related to 'passaic-class monitor' with the relationship of 'operator'.  Choices: - southern  - united states navy
The answer is:
united states navy