Phibun's alliance with Japan during the early years of war was initially popular. The Thai army joined Japan's Burma Campaign with the goal of recovering part of the Shan states previously given to Britain by the Treaty of Yandabo. They gained the return of the four northernmost Malay states lost in the Anglo-Siamese Treaty of 1909, and with Japanese mediation in the Franco–Thai war they also recovered territory lost in the Franco-Siamese War of 1893.