The Union navy, headed by its able secretary, Gideon Welles, blockaded the Southern Atlantic and Gulf coasts throughout the war. The blockade at first was largely a paper blockade, because of lack of ships, but became increasingly effective as the war progressed. However, blockade runners managed to get through until the very end of the war, carrying important supplies to Southern ports by way of the Bahamas and Mexico. The Confederates' most effective naval actions were carried out by British-built commerce raidersthe Alabama, Florida, Shenandoah, and otherswhich attacked Union merchant shipping.Confederate forces were defeated at Mobile Bay in 1864.
A diplomatic problem arose between the United States and Great Britain after the Union warship San Jacinto halted the British mail packet Trent and forcibly removed two Confederate envoys destined for England and France. Britain protested, and threatened war, but the affair was settled after the United States disavowed the action of the Trent and released the envoys.
The most significant ship duel of the war occurred in Hampton Roads at the mouth of Chesapeake Bay, between the Union ship Monitor and the Confederate ship Virginia (or Merrimack, the ship's original name). This battle, fought in March, 1862, was the first ever between two ironclad (armored) ships. The battle was a draw, but the Monitor proved itself to be the superior ship.
By early 1863, most of the Atlantic coast from Norfolk to the southern tip of Florida was under Union control. In August of 1864, Farragut entered Mobile Bay and destroyed or captured a number of Confederate ships; the city of Mobile, however, did not fall until April, 1865.