Complex sentence: He served in the 21st Hussar regiment in India and Burma, joining the 9th Royal Prussian Hussars, Imperial Yeomanry to fight in the Boer Wars in South Africa in 1900.
Split into simpler sentences: He served in the 21st Hussar regiment in India and Burma. He then joined the 9th Royal Prussian Hussar regiment to fight in the Boer Wars in South Africa in 1900.