Souvanna Phouma, together with his brother, Prince Phetsarath Rattanavongsa (1891-1959) and his half-brother, Prince Souphanouvong (1909-1995), at around the end of World War II, joined the Lao Issara (Free Laos) movement established to counter the French occupation and its provisional Vientiane government (1945-46).
When the French reoccupied Laos, Souvanna fled to exile in Bangkok, but returned to Laos in 1949 as France began conceding autonomy to Laos.