Complex sentence: In the late 13th century, Palestine and Syria became the primary front against the fast-expanding Mongol Empire, whose army reached Palestine for the first time in 1260, beginning with the Mongol raids into Palestine under Nestorian Christian general Kitbuqa.
Split into simpler sentences: In the late 13th century, Palestine and Syria became the primary front against the fast-expanding Mongol Empire. The Army of the Mongol Empire reached Palestine for the first time in 1260, beginning with the Mongol raids into Palestine under Nestorian Christian general Kitbuqa.