Complex sentence: Due to the French inability to establish administrative control, the frontier between Syria and Palestine was fluid until 1920, when the French managed to asserted authority over the Arab nationalist movement and King Faisal had been deposed.
Split into simpler sentences: Due to the French inability to establish administrative control the frontier between Syria and Palestine was fluid. Until, 1920, when the French managed to asserted authority over the Arab nationalist movement and King Faisal had been deposed.