This was a US eight-day final training exercise for the army and navy forces (the latter comprising Force ‘U’) allocated to the 'Utah' Beach component of ‘Overlord’ (22/30 April 1944).
The exercise was to involve a practice landing on the Slapton Sands in southern Devon. Loaded on board tank landing ships, 30,000 US servicemen of Major General Joseph L. Collins’s VII Corps of Lieutenant General Omar N. Bradley’s 1st Army prepared their simulated assault landing as the tank landing ships approached the Slapton Sands across Lyme Bay. However, the force was spotted by nine S-boote operating out of Cherbourg, and in their attack the German fast attack craft used torpedoes and gunfire to sink two of the transports and set a third ablaze. The event caused the deaths 749 US servicemen (551 and 198 from the US Army and US Navy respectively), whereas the real 'Utah' Beach landing resulted in only some 200 casualties.
Concerned about the effects any public revelation of this disaster would have shortly before the real invasion was launched, the Allied authorities swore to secrecy all the survivors. Ten missing officers involved in the exercise had clearance at ‘Bigot’ level for ‘Overlord’, meaning that they knew the invasion plans and could have compromised the invasion should they have been captured alive. As a result, ‘Overlord’ was nearly called off until the bodies of all 10 victims had been found.
Several changes resulted from mistakes made in ‘Tiger’ (ix): radio frequencies were standardised as the tank landing ships' British escort vessels were late and out of position as a result of radio problems; better life jacket training for landing troops; and new plans for small craft to pick up floating survivors on D-Day.
The disaster was kept secret at the time, as noted above, but also after the war, and became public knowledge only in the 1980s.