When the torpedo struck, the sea was calm and the spring weather warm. But the passengers on the Lusitania, Cunard’s leading transatlantic liner, were soon to become victims of one of the most infamous and dramatic atrocities of the first world war. The sinking of the great ship, which for a time was the largest passenger vessel ever built, broke all international wartime agreements and resulted in the death of 1,198 people. The political reaction changed the path of the conflict.

After the tragedy, Able Seaman Joseph Parry was presented with a medal for gallantry by George V. Photograph: National Museums Liverpool;

Among Cunard’s crew was Able Seaman Joseph Parry and now, 100 years on, the story of his bravery is to be told for the first time in a new exhibition in Liverpool this spring. Working in the sea without rest after the German torpedo struck, Parry managed to save 100 people. He was later awarded a Board of Trade gallantry medal by George V at Buckingham Palace.

Next month, his descendants will lend the Merseyside Maritime Museum not only this medal, but also the baby’s bootie Parry was given by the grateful mother of an infant whom he helped to rescue. On the sole, still visible, Parry wrote: “Lest we forget Lusitania 7 May 1915”.

“It is wonderful to be able to display the little boot, along with other poignant items that have never been shown to the public before, and to let people hear about Parry’s story,” said museum curator Eleanor Moffat. “The Lusitania was loved by Liverpudlians … and news of its sinking went around the globe within a day. People today know about the Titanic, but at the time, and certainly for people in Liverpool and New York, this was just as terrible a disaster. Since more than 100 Americans were among the dead, it also influenced America’s decision to come into the war two years later. ”

Before the Lusitania sailed from New York on 1 May 1915, passengers had been warned that enemy action was possible. U-boats had begun to target merchant vessels and that February Germany had declared the sea around Britain a war zone. As the ship reached the coast of Ireland on 7 May, a torpedo struck its starboard side, followed by an internal explosion and the ship soon began to list heavily. It took just 18 minutes to sink.

Parry worked with another Able Seaman, Leslie Morton, to pluck survivors from the sea. They were joined by Irish fishermen who had heard of the incident. The wreck lies 11 miles off the Old Head of Kinsale lighthouse in 300ft of water.

Such loss of life is not the only reason the Lusitania is remembered. Controversy hasalso centred on rumours that the Clydeside-built vessel was deliberately allowed to sail into danger in the hope of drawing America into the conflict. Historians have found evidence that Winston Churchill, then First Lord of the Admiralty, certainly hoped that the perception of international peril might increase. A week before the sinking, he wrote to the president of the Board of Trade, urging him it was “most important to attract neutral shipping to our shores, in the hope especially of embroiling the United States with Germany”.

While passengers were warned in New York by a grim sign forwarded from the German embassy and placed next to details of the return journey to Liverpool, most would not have known that a large shipment of weapons components would be in the hold. The Lusitania was officially listed as an auxiliary warship and its cargo included rifle cartridges, fuses and shell cases, openly listed on the cargo manifest.

To mark the centenary a walk of remembrance will take place from the church to the quayside between maritime museum and Museum of Liverpool, where Lusitania’s propeller is displayed.

“We are working with Liverpool Parish Church and Liverpool City Council to ensure we remember those lost in a dignified way. There are so many families living in Merseyside whose descendants were aboard Lusitania when it was lost,” said Ian Murphy, deputy director of the maritime museum.

In May Cunard’s MS Queen Victoria will also make a voyage to Cork, Ireland, in commemoration of the loss of Lusitania.