The first and second systems of seacoast defenses in the United States marked the beginning of Americans' effort to defend their coastlines from foreign invasion. However, these systems were not standardized in material or design. The third system represented the first real effort by the Federal government to form a permanent defensive system, which would be consistent in design and execution throughout the country.
The first two systems of seacoast defenses were built at a time when the country felt an immediate threat, causing them to be built rapidly and leading to failures during the War of 1812. However, the peace that followed the war gave the United States time to plan, design, and build the forts of the third system. To design the forts and supervise construction, the War Department established a board of officers, originally led by French engineer General Simon Bernard and later by American Lt. Colonel Joseph G. Totten.
The principle forts of the third system, which were built between 1818 and 1867, evolved over time adopting the latest military technology and varied based on the constraints of the waterway each fort was meant to protect. However certain defining features distinguished third system forts including: high vertical walls, masonry or stone construction, and casemated cannon emplacements.
Although fifty defensive positions were planned in the original 1821 report, only 34 of the forts were completed by the time the system became obsolete. Those that were built never got the opportunity to serve their original purpose to protect the most important harbors and waterways of the United States. At the outbreak of the Civil War in 1861 these forts rose to fame as two American armies fought one another, including forts Monroe, Macon, Pulaski, Clinch, Pickens, Jefferson, and Sumter where the first shots of the Civil War were fired.
Serving capably in the early months of the war, the forts of the third system did not last long after the widespread adoption of the rifled cannon. With their increased range and accuracy, rifled cannon could breach the solid masonry walls with incredible efficiency. As the war dragged on, many third system forts were reinforced with earthen mounds for added protection, against the new artillery.
Following the Civil War, as the Industrial Revolution exploded around the world, the pace of artillery advancement increased rapidly. The forts of the third system were soon obsolete. By 1885 a new board of officers was established to replace the failing third system of American seacoast defenses. This new system would be known as the fourth system of American seacoast defenses.