A bloody invasion by Great Britain changed Tibet's fortunes forever.  In a story in the New York Times, the 1904 invasion of Tibet by Britain's Sir Francis Younghusband pulled off a ploy for personal glory and gained commercial concessions for his government. He stayed for 2 months and destroyed the medieval Tibetan army.  These events drove the Chinese to occupy Tibet as a buffer to other colonial incursions.  The story of the events are detailed in a book by Charles Allen titled "Duel in the Snows", which is also available in the OBG/Amazon bookstore.