I would say the crusade to blame for the decline of Muslims golden age. When the crusade started around 12 century, it brought devision among Muslims and Christ

I would say the crusade to blame for the decline of Muslims golden age. When the crusade started around 12 century, it brought devision among Muslims and Christians in places such as Iraq, Syria, Lebanon and Palestine. For centuries, Muslims, Christians and Jews lived next to each other and collaborated with each other but the crusade killed that friendship.