RMS Lusitania (named after the Roman province in Western Europe corresponding to modern Portugal) was a British ocean liner that was launched by the Cunard Line in 1906 and that held the Blue Riband appellation for the fastest Atlantic crossing in 1908. It was briefly the world's largest passenger ship until the completion of the Mauretania three months later. She was sunk on her 202nd trans-Atlantic crossing, on 7 May 1915, by a German U-boat 11 miles (18 km) off the southern coast of Ireland, killing 1,198 passengers and crew.The sinking occurred about two years before the United States declaration of war on Germany. Although the Lusitania's sinking was a major factor in building American support for a war, war was eventually declared only after the Imperial German Government resumed the use of unrestricted submarine warfare against American shipping in an attempt to break the Transatlantic supply chain from the US to Britain, as well as after the Zimmermann Telegram.

German shipping lines were Cunard's main competitors for the custom of Transatlantic passengers in the early 20th century, and Cunard responded by building two new 'ocean greyhounds': the Lusitania and the Mauretania. Cunard used assistance from the British Admiralty to build both new ships, on the understanding that the ship would be available for military duty in time of war. During construction gun mounts for deck cannons were installed but no guns were ever fitted. Both the Lusitania and Mauretania were fitted with turbine engines that enabled them to maintain a service speed of 24 knots (44 km/h; 28 mph). They were equipped with lifts, wireless telegraph, and electric light, and provided 50 per cent more passenger space than any other ship; the first-class decks were known for their sumptuous furnishings.The Royal Navy had blockaded Germany at the start of the First World War; the UK had declared the North Sea a war zone in the autumn of 1914 and mined the approaches. In the spring of 1915, all food imports for Germany were declared contraband. In response Germany had declared the seas around the United Kingdom a war zone too, and German submarine warfare was intensifying in the Atlantic. When RMS Lusitania left New York for Britain on 1 May 1915 the German embassy in the United States placed fifty newspaper advertisements warning people of the dangers of sailing on Lusitania. Objections were made by the British that threatening to torpedo all ships indiscriminately was wrong, whether it was announced in advance or not.On the afternoon of 7 May, a German U-boat torpedoed Lusitania 11 miles (18 km) off the southern coast of Ireland inside the declared war zone. A second internal explosion caused her to sink in 18 minutes, killing 1,198 passengers and crew. The German government justified treating Lusitania as a naval vessel because she was carrying 173 tons of war munitions and ammunition, making her a legitimate military target, and they argued that British merchant ships had violated the cruiser rules from the very beginning of the war. The internationally recognised cruiser rules were obsolete by 1915; it had become more dangerous for submarines to surface and give warning with the introduction of Q-ships in 1915 by the Royal Navy, which were armed with concealed deck guns. The Germans argued that Lusitania was regularly transporting "war munitions"; she operated under the control of the Admiralty; she could be converted into an armed auxiliary cruiser to join the war; her identity had been disguised; and she flew no flags. They claimed that she was a non-neutral vessel in a declared war zone, with orders to evade capture and ram challenging submarines.However, the ship was not armed for battle and was carrying hundreds of civilian passengers, and the British government accused the Germans of breaching the cruiser rules. The sinking caused a storm of protest in the United States because 128 American citizens were among the dead. The sinking shifted public opinion in the United States against Germany and was one of the factors in the declaration of war nearly two years later. After the First World War, successive British governments maintained that there were no munitions on board Lusitania, and the Germans were not justified in treating the ship as a naval vessel. In 1982, the head of the Foreign Office's American department finally admitted that there is a large amount of ammunition in the wreck, some of which is highly dangerous and poses a safety risk to salvage teams.

Cunard Line () is a British cruise line based at Carnival House at Southampton, England, operated by Carnival UK and owned by Carnival Corporation & plc. Since 2011, Cunard and its three ships have been registered in Hamilton, Bermuda.In 1839, Samuel Cunard was awarded the first British transatlantic steamship mail contract, and the next year formed the British and North American Royal Mail Steam-Packet Company in Glasgow with shipowner Sir George Burns together with Robert Napier, the famous Scottish steamship engine designer and builder, to operate the line's four pioneer paddle steamers on the Liverpool–Halifax–Boston route. For most of the next 30 years, Cunard held the Blue Riband for the fastest Atlantic voyage. However, in the 1870s Cunard fell behind its rivals, the White Star Line and the Inman Line. To meet this competition, in 1879 the firm was reorganised as the Cunard Steamship Company, Ltd, to raise capital.In 1902, White Star joined the American-owned International Mercantile Marine Co. In response, the British Government provided Cunard with substantial loans and a subsidy to build two superliners needed to retain Britain's competitive position. Mauretania held the Blue Riband from 1909 to 1929. Her running mate, Lusitania, was torpedoed in 1915 during the First World War.

In 1919, Cunard relocated its British homeport from Liverpool to Southampton, to better cater for travellers from London. In the late 1920s, Cunard faced new competition when the Germans, Italians and French built large prestige liners. Cunard was forced to suspend construction on its own new superliner because of the Great Depression. In 1934, the British Government offered Cunard loans to finish Queen Mary and to build a second ship, Queen Elizabeth, on the condition that Cunard merged with the then ailing White Star line to form Cunard-White Star Line. Cunard owned two-thirds of the new company. Cunard purchased White Star's share in 1947; the name reverted to the Cunard Line in 1950.Upon the end of the Second World War, Cunard regained its position as the largest Atlantic passenger line. By the mid-1950s, it operated 12 ships to the United States and Canada. After 1958, transatlantic passenger ships became increasingly unprofitable because of the introduction of jet airliners. Cunard undertook a brief foray into air travel via the "Cunard Eagle" and "BOAC Cunard" airlines, but withdrew from the airliner market in 1966. Cunard withdrew from its year-round service in 1968 to concentrate on cruising and summer transatlantic voyages for holiday makers. The Queens were replaced by Queen Elizabeth 2 (QE2), which was designed for the dual role.In 1998, Cunard was acquired by the Carnival Corporation, and accounted for 8.7% of that company's revenue in 2012. In 2004, QE2 was replaced on the transatlantic runs by Queen Mary 2 (QM2). The line also operates Queen Victoria (QV) and Queen Elizabeth (QE). As of 2019, Cunard is the only shipping company to operate a scheduled passenger service between Europe and North America.