Attack transports, a variant of ocean-going troopship adapted to transporting invasion forces ashore, carry their own fleet of landing craft. Landing ships beach themselves and bring their troops directly ashore.
Ships to transport troops were already used in Antiquity. Ancient Rome used the navis lusoria, a small vessel powered by rowers and sail, to move soldiers on the Rhine and Danube.
The modern troopship has as long a history as passenger ships do, as most maritime nations enlisted their support in military operations (either by leasing the vessels or by impressing them into service) when their normal naval forces were deemed insufficient for the task. In the 19th century, navies frequently chartered civilian ocean liners, and from the start of the 20th century painted them gray and added a degree of armament; their speed, originally intended to minimize passage time for civilian user, proved valuable for outrunning submarines and enemy cruisers in war. HMT Olympic even rammed and sank a U-boat during one of its wartime crossings. Individual liners capable of exceptionally high speed transited without escorts; smaller or older liners with poorer performance were protected by operating in convoys.
Most major naval powers in the late 19th and early 20th centuries provided their domestic shipping lines with subsidies to build fast ocean liners capable of conversions to auxiliary cruisers during wartime. The British government, for example, aided both Cunard and the White Star Line in constructing the liners RMS Mauretania, RMS Aquitania, RMS Olympic and RMS Britannic. However when the vulnerability of these ships to return fire was realized during World War I most were used instead as troopships or hospital ships.
RMS Queen Mary and RMS Queen Elizabeth were two of the most famous converted liners of World War II. When they were fully converted, each could carry well over 10,000 troops per trip. Queen Mary holds the all-time record, with 15,740 troops on a single passage in late July 1943, transporting a staggering 765,429 military personnel during the war.
Large numbers of troopships were employed during World War II, including 220 "Limited Capacity" Liberty ship conversions, 30 Type C4 ship-based General G. O. Squier-class, a class of 84 Victory ship conversions, and a small number of Type-C3-S-A2 ship-based dedicated transports, and 15 classes of attack transports, of which some 400 alone were built.
The modified Liberties were capable of transporting up to 450, 550, or 650 (sources vary) troops or prisoners-of-war. Modifications included installation of bunks stacked five deep on the forward tweendeck, additional shower and head facilities, two additional diesel-powered generators, and installation of two more Oerlikon 20-mm automatic cannons.
30 Type C4 ship-based General G. O. Squier-class, the largest carrying over 6,000 passengers.
A class of Victory ship-based dedicated troopship was developed late in World War II. A total of 84 such VC2-S-AP2 hull conversions was completed.
A class of Type C3 ship comprising mainly C3-S-A2 and C3-S-A3 hulls was also converted to dedicated troopships, capable of carrying 2,100 troops, was also developed.
At least 15 classes of Attack Transport, consisting of at least 400 ships specially equipped for landing invasion forces rather than general troop movement.