A plaque is being dedicated as part of an Armed Forces Day in Holyhead to commemorate 26 sailors who died when their submarine sank in 1918.
HMS H5 was mistaken for a German U-boat and rammed by a British merchant ship off the north Wales coast.
The merchant ship, the Rutherglen, later docked at Holyhead, Anglesey.
Other events in the town on Saturday include a military parade and a visit from a Royal Navy minehunter.
All 26 sailors on HMS H5 lost their lives, including a US lieutenant who was an observer on board and was the first member of the US Navy to die in World War I.
The Rutherglen later docked at Holyhead and reported sinking a German U-boat.
HMS H5 was overdue by four days and it became apparent that the merchant ship had actually struck a British submarine.
The reality of what had happened was not revealed to the crew of the Rutherglen or the families of those who died.
It was some 50 years later before the tragic circumstances emerged.
Safeguarding the ship
A dedication service is also taking place for a seaman from Amlwch on Anglesey, who was part of the crew of HMS Pargust in July 1917.
Seaman William Williams played a key role in safeguarding the ship from an onslaught by a German U-boat
The U-boat, thinking the ship was a merchant vessel, surfaced and the captain on HMS Pargust gave the order to fire, sinking the submarine.
The crew's efforts were recognised with a Victoria Cross, and a ballot was held to see which members of the crew would collect the medal.
Seaman Williams was one of those who was put forward.
The Army, RAF and Royal Navy are involved in a series of events for Armed Forces Day in Holyhead on Saturday.
Following a Spitfire fly past and a flag-raising ceremony, there will be a military parade through the town.
Other events are taking place around the UK this weekend and next weekend.
Armed Forces Day was established to recognise the work and bravery of all serving troops, veterans, recruits and their families.
Cardiff hosts the second national Armed Forces Day next Saturday, 26 June.