Liberty ships were cargo ships built in the United States during World War II. Though British in conception, they were adapted by the U.S. as they were cheap and quick to build, and came to symbolize U.S. wartime industrial output. Based on vessels ordered by Britain to replace ships torpedoed by German U-boats, they were purchased for the U.S. fleet and for lend-lease provision to Britain. Eighteen American shipyards built 2,751 Libertys between 1941 and 1945, easily the largest number of ships produced to a single design.

The production of these vessels mirrored, on a much larger scale, the manufacture of the Hog Islander ship and similar standardized types during World War I. The immense effort to build Liberty ships, the sheer number of ships built, and the fact that some of the ships survived far longer than the original design life of five years, make them the subject of much study.

In 1936, the American Merchant Marine Act was passed to subsidize the annual construction of 50 commercial merchant vessels to be used in wartime by the United States Navy as naval auxiliaries. The number was doubled in 1939 and again in 1940 to 200 ships a year. Ship types included a tanker and three types of merchant vessel, all to be powered by steam turbines. Limited industrial capacity, especially for reduction gears, meant that relatively few of these ships were built.

Captain Jeremiah O’Brien (1744–1818) was a captain in the Massachusetts State Navy. Prior to its existence (or that of the Continental Navy, he commanded the sloop Unity when she captured HMS Margaretta in the Battle of Machias, the first naval battle of the American Revolutionary War.

He was born at Kittery, Maine. O'Brien and his five brothers, Gideon, John, William, Dennis and Joseph, were crewmembers of the sloop Unity when she captured Margaretta on June 12, 1775, at the entrance to the harbor at Machias (a town then in Massachusetts, now in Maine).

Under the command of Jeremiah O’Brien, thirty-one townsmen armed with guns, swords, axes, and pitch forks captured the British armed schooner in an hour-long battle after Margaretta had threatened to bombard the town for interference with the shipment of lumber to British troops in Boston, Massachusetts. This battle is often considered the first time British colors were struck to those of the United States, even though the Continental Navy did not exist at the time. The United States Merchant Marine claims Unity as its member and this incident as their beginning.