Complex sentence: Using tactics similar to its economic marriage to Iran, Britain took Tibet under its wing by first invading it in 1903 and then making it a trade partner, ultimately allowing Tibet to accumulate a large debt and forfeit even more power over to Britain.
Split into simpler sentences: Using tactics similar to its economic marriage to Iran, Britain took Tibet under its wing by first invading it in 1903 and then making it a trade partner. It ultimately allowed Tibet to accumulate a large debt and forfeit even more power over to Britain.