In early 1975 the underlying tension between the claimants came to the surface when South Vietnam invaded Southwest Cay, then occupied by the military forces of its wartime ally, the Philippines.
By the time the Philippines military were able to put their plan into action, the South Vietnamese had already built a strong defense on Southwest Cay, thus deterring any potential counter-attack.
The operation would commence at 12.00 am on 9 April 1975, to coincide with the PAVN ground attack on Xuân Lộc and to take advantage of low tide to land the special forces on the South Vietnamese-occupied sections of the Spratly Islands.
[4] Unlike their North Vietnamese counterparts, South Vietnam possessed a large naval force with a strong fleet of ships.
[7] On 9 April 1975, three VPN transport ships, disguised as fishing trawlers, began moving towards Southwest Cay in the Spratly Islands with members of the 126th and 471st Battalions all on board.
Having achieved their initial objective, the VPN command sent out the transport ship T641, to carry all the captured ARVN soldiers back to Da Nang.
On the night of 24 April, under the observation of the Taiwanese military, VPN transport ships sailed in a single column passed the Taiwanese-occupied island of Itu Aba towards their next objective.
As a result, at 8.45 pm on 26 April, RVN ships in the Spratly Island area were ordered to evacuate the ARVN 371st Local Battalion and withdraw from Namyit and Sin Cowe.
At the end of March 1975, the ARVN maintained a security team on Phú Quý, which included one police platoon and 4,000 members of the People's Self-Defense Forces.
[15][16] Côn Đảo archipelago is located in the southwestern area of the South China Sea, nearly 180 kilometers (110 mi) from the city of Vũng Tàu.
On 29 April, the airfield at Côn Sơn became a staging post where South Vietnamese government officials and U.S. advisers were assembled, to be evacuated to the U.S. warships of the 7th Fleet which anchored nearby.
They set up a Provisional Committee to govern the island, and organised three platoon-sized units using captured weapons to march on the remnants of the ARVN.
By 8 am the prisoners had captured all former South Vietnamese infrastructure and assets, including 27 aircraft, as the remaining ARVN soldiers at the Côn Sơn airport also surrendered.
[18][19] On the evening of 2 May, the rebel prisoners on Côn Đảo Island successfully established communications with North Vietnamese military units.
Throughout the day regular North Vietnamese military units and the rebel prisoners coordinated to establish control over the rest of the Côn Đảo Archipelago.
[18][20] After more than two months of planning and combat operations, the VPN successfully captured the Spratly, Phú Quý and Côn Đảo groups of islands from the South Vietnamese.