On 17 January 1941 there was a naval battle off Koh Chang between the Thai and French navies. This was part of the Franco-Thai war which involved France trying to take control of various territories that were then part of Thailand. All this whilst Thailand was technically neutral and France was then on Japan’s side in this part of the world during WWII.
Every year on the anniversary a ceremony is held to remember the Thai sailors who lost their lives during the battle. This is held at the memorial at Hat Yuthanavy beach in the south east of the island, near Long beach. This is the closest point to where one of the Thai ships sank. Wreaths are laid on the buoy which marks the ships location.
These photos are provided by the Koh Chang Administration: https://www.facebook.com/kohchangtai