A troopship is a ship which has been outfitted for the purpose of transporting military troops in either peace or wartime. Many nations have relied heavily on commandeered ships for troop transport historically, with some governments even subsidizing domestic shipping industries in exchange for a pledge on the part of shipping companies to allow the military to use their ships as needed. Troopships were extensively used during the First and Second World Wars to transport huge numbers of troops to the Pacific and European theaters, and they continue to be used, along with aircraft, on a smaller scale.
Many nations prefer to use converted passenger liners as troopships. There are several reasons for converting rather than building customized ships for the purpose of transporting troops. The first reason is that a troopship does not require specialized equipment, and it may only need to be used on a limited number of occasions, making the commandeering and conversion of an existing ship more cost effective than building transport from scratch. The potential for conversion may also be touted as a justification for providing subsidies to the ship building industry.
Using converted passenger ships can also be convenient, since these ships are designed to carry large numbers of people and achieve high speeds. Some converted troopships in the First and Second World Wars even managed to sink the enemy, with one ship notably ramming an enemy destroyer. Using privately owned ships also reduces maintenance costs for the military, which can be a definite bonus in the eyes of paper pushing administrators.
In addition to carrying troops, a troopship may also be used to carry some military materiel, including vehicles, medical supplies, and so forth, assuming that the cargo hold of the ship is sufficiently large. Some troopships have also specialized as hospital ships, providing transport and medical care to injured members of the military. For converted ships which are slow and vulnerable to attack, serving as a hospital ship may be preferable to serving as a troopship, since hospital ships are supposed to be immune from enemy attacks.
Some very famous ocean liners have served as troopships, including the Mauritania, the infamous Lusitania's sister ship, along with the Queen Mary, Queen Elizabeth II, Leviathan, and the doomed Titanic's sister ships Olympic and Britannic. In some cases, the fittings of these ships were left largely intact while the exteriors were painted gray or ornamented with dazzle camouflage, which must have been a surreal experience for soldiers sailing to war on famous luxury liners used as troopships.