The Belgian liner LEOPOLDVILLE had been engaged in the Antwerp and Congo service and was taken over as a troopship when Belgium was invaded by Germany in 1940. On the 24th December 1944 she was ferrying 2,235 men of the US’s Army 66th Division (The Black Panthers) from Southampton to Cherbourg to reinforce troops fighting the Battle of the Bulge. This was the 23rd such passage the vessel had made.
Along with another troopship, HMS CHESHIRE, she was guarded by the destroyers HMS BLILLIANT,

HMS ANTHONY and HMS HOTHAM and the Free French Frigate CROIX de LORRAINE
Despite the fact that U boats were known to be in the area there was no air cover and the accompanying escorts were operating on a different radio frequency from the US base on the French shore
At 5.55pm the German submarine U486 (Lt G Meyer) torpedoed the LEOPOLDVILLE hitting her on the starboard side.
The explosion caused the two after decks of the ship to collapse trapping a large number of men . The Belgian Captain, Charles Limbor, did not appreciate the seriousness of the situation and dropped anchor 5½ miles from Cherbourg Harbour to save her drifting, possibly into a minefield, believing that the transport was in no immediate danger.
Three quarters of an hour elapsed before the work of disembarkation began however  none of the troops on board had received lifeboat drill or were shown how to fasten their lifebelts,
HMS BRILLIANT stood by while the other vessels in the escort went in search of the submarine.
At 6.26pm HMS BRILLIANT using a signal lamp contacted the shore but failed to respond when asked what kind of assistance was needed. The escorts did report the action to Dover by radio but it took more than an hour for news of the attack to reach Cherbourg via a telephone call.
At 7.30pm HMS BRILLIANT embarked 500 of those on LEOPOLDVILLE and took them to Cherbourg it still being thought that the transport was in no immediate danger.
A heavy sea meant that when HMS BRILLIANT had came alongside LEOPOLDVILLE, due to the disparity between the deck height of the vessels (see photo), it was difficult for the troops to jump between the two decks. Many soldiers fell between them and were either crushed or drowned. Soldiers who jumped overboard were weighed down by their heavy clothing and equipment and had little chance of survival in the cold water.
Troops left on board had been assured that the ship wasn’t sinking but saw the Belgian crew fleeing with packed suitcases in half empty lifeboats. Owing to a mistaken order part of the crew had abandoned ship leaving the unfortunate soldiers to launch the remaining boats as best they could. In all 802 troops and six of the crew including the captain were drowned.
HMS BRILLIANT returned to the scene but the LEOPOLDVILLE had sunk The ship had remained afloat until 8.30pm when two further explosions were heard, the bulkheads gave way and she keeled over and sank within 10 minutes
The troops had displayed amazing bravery by remaining calm and disciplined waiting for orders. By the end 802 troops and six of the crew including the captain were drowned.
Small craft from Cherbourg arrived too late and only a few men were rescued from the sea. This was the greatest single loss suffered by US Servicemen at sea
Captain Charles Limbor was the only officer who did not survive the disaster.
The British and American governments hushed up the tragedy. The families of those who died were informed only that they were missing in action. Many never learnt the truth
The U486 a Type VIIC submarine with a crew of 44 was torpedoed on the 12th April 1945 North West of Bergen by the submarine HMS TAPIR (Lt-Commander Roxburgh) with the loss of all hands

A full account of the tragedy can be found in the book “The SS Leopoldville Disaster, December 24, 1944” by Allan Andrade published by Tern Book Co. ISBN-10: 1890309540 ISBN-13: 978-1890309541