Japanese conquest of Burma main attack westward from Rahaeng in Thailand across the Kawkareik Pass. The 16th Indian Infantry Brigade of the 17th Indian Division guarding this approach retreated hastily westward. The Japanese division advanced to Moulmein at the mouth of the Salween River which was garrisoned by the 2nd Burma Infantry Brigade. The position was almost impossible to defend, and had the River Salween, almost wide, behind it. The 2nd Burma Brigade was squeezed into a progressively tighter perimeter, and eventually retreated over the river by ferry on 31 January after abandoning a large amount of supplies and equipment. Part of the force
Sai River (Thailand) flowing through the Tachileik/Mae Sai urban area, where there is a sign marking the northern most point of Thailand just east of the border-crossing bridge —at the meeting point of Thailand Route 1 and National Highway 4 (Burma)— over the river. Finally it flows northeastwards before joining the right side of the Ruak River, a tributary of the Mekong, less than southeast of Tachileik Airport. Sai River (Thailand) Sai River (; ; ), formerly known as the River of Lawa (; ), is a river that forms the natural border between Thailand and Myanmar at Tachileik and Mae Sai Districts.