Following Japan's entry into Indochina on 22 September 1940, the Thai government, under the pro-Japanese leadership of Field Marshal Plaek Phibunsongkhram, and strengthened by virtue of its treaty of friendship with Japan, invaded French Protectorate of Cambodia's western provinces to which it had historic claims. Following the Franco-Thai War, Tokyo hosted the signature of a treaty on 9 May 1941 that formally compelled the French to relinquish one-third of the surface area of Cambodia with almost half a million citizens.