Final answer:
The primary duty of the Navy during the Civil War was to enforce a blockade of Confederate ports, thereby restricting the Confederacy's trade and supply lines, which was vital for the Union's overall war strategy.
Step-by-step explanation:
The primary duty of the Navy during the Civil War was to implement and maintain a blockade of Confederate ports, which was a key aspect of the Union's Anaconda Plan to choke off supplies to the South. This blockade prevented the Confederacy from exporting cotton and importing much-needed supplies and military equipment. The Union Navy also had duties that included riverine warfare, particularly on the Mississippi River, and supporting ground forces by providing artillery support and transporting troops.
The implementation of this naval blockade was a strategic move initiated by President Abraham Lincoln shortly after the fall of Fort Sumter on April 15, 1861. The goal was not just to suppress insurrections but also to directly impact the Confederacy's ability to sustain the war effort. The blockade was a significant factor in the eventual Union victory, as it crippled the Confederate economy and limited its long-term warfighting capabilities.