Complex sentence: The Afghan Arabs served as the essential core of the foot soldiers of Osama bin Laden's Al Qaeda, bin Laden being seen, according to journalist Lawrence Wright, as "the undisputed leader of the Arab Afghans" by fall of 1989.
Split into simpler sentences: The Afghan Arabs served as the essential core of the foot soldiers of Osama bin Laden's Al Qaeda. Bin Laden being "the undisputed leader of the Arab Afghans" by fall of 1989.