Shortly after the American Civil War broke out in 1861, the Confederacy quickly began developing an ironclad based on the hull of the USS Merrimack which had been burned by Federal troops before the naval base at Norfolk - Gosport Navy Yard - had been captured by the recently seceded Commonwealth of Virginia. The United States Congress addressed this issue in August 1861 and recommend that armored ships be built for the Union Navy. Ericsson still had a dislike of the U.S. Navy but he was convinced by Cornelius Scranton Bushnell to work on an ironclad for them. Ericsson presented drawings of the USS Monitor a totally unique and novel design of armoured ship, which after much controversy was eventually built and finished on March 6, 1862. The ship went from plans to launch in approximately 100 days, an amazing achievement.

The infantry in the American Civil War' comprised foot-soldiers who fought primarily with small arms, and they carried the brunt of the fighting on battlefields across the country. As the Civil War progressed, battlefield tactics soon changed in response to the new form of warfare being waged in America. The use of military balloons, rifled muskets, repeating rifles, and fortified entrenchments contributed to the death of many men. Officers, many professionally trained in tactics from the Napoleonic Wars, were often slow to develop changes in tactics in response.

As a private citizen Parrott was able to experiment with cannons and projectiles without the usual red tape involved in government foundries. His accomplishments during his tenure included the perfection of a rifled cannon and its corresponding projectile (both named after him) patented in 1861, and the Parrott sight and fuze which were developed during the Civil War years.