 However, following the fall of the Abbassid dynasty, the flow of Arab immigrants into Persia gradually diminished, but it nonetheless continued.From 1219 to 1335, Mongols under Genghis Khan and then his grandson Hulegu invaded from the steppes to the east of the Iranian plateau, reaching Palestine before they were stopped.Baghdad, the rich and fabled capital of the Abbasid caliphs, fell in 1258.