A brief(ish) history of Burma and Myanmar

Today, we had a speaker come in to tell us about the history of Myanmar (formally Burma).

He started off the speech with an interesting quote; "Mynamar. An enchanted country. Enchanted, but benighted." In 1820, Myanmar, which translates to fast and strong, started appeared to be quite the opposite, when Britain declared war on Burma. The British won, and Burma was now under British control. In 1890, our speaker's grandfather arrived in Burma. By 1930, life in Burma was idyllic - especially if you were white. But things took a turn in 1941, when Japan invaded. However, in 1945, Britain regained control of Burma and General Aung San signed a treaty. This treaty brought together the tree tribes.

Now I leave you with one queston: How did the tree tribes and the main population seperate? And why?