Some pictures from the Battle of Insein 1949 between the Burmese government and the Karen forces, just North of Naypyidaw

The TLDR is that the Karen, which had been Britain's favorite ethnic group in Burma, revolted against the new Burmese government due to the latter refusing to grant them autonomy.

The Karen upon their revolt overwhelmed the Burmese government forces which were. The difference in quality of their forces and officers was stark.

The Karen had a long standing martial culture and due to the British preferring them they made up a disproportionate amount of the pro-British Burmese forces.

Via British training the Karen had the best trained soldiers and offices in the country

In the end it took a coalition of the Burmese government, the Burmese communists, which themselves were at war with the government, and all other loyal ethnic groups to repel the Karen forces and lift the siege of the capital

Karen forces retreated into their ethnic homelands and continued their insurgency to this day

Some pictures of French volunteers/mercenaries alongside Karen rebel forces

During the cold war it became popular for former soldiers and spec ops from France and the Anglosphere countries to travel to Burma and fight alongside the Karen rebels