The Crusades is a possible answer. They paved the way for much cultural transmission. Trading can be another possible answer.
Explanation:
A river formed by the confluence of the Allegheny and Monongahela Rivers in western Pennsylvania and flowing about 1,580 km (980 mi) to the Mississippi River at Cairo in southern Illinois. The British and French contested control of the river until 1763.
After Muhammad's death in 632 CE, his friend Abu Bakr was named caliph and ruler of the Islamic community, or Ummah. Sunni Muslims believe that Abu Bakr was the proper successor, while Shi'a Muslims believe that Ali should have succeed Muhammad as caliph.
The Rashidun Caliphate (632–661) was led by Abu Bakr, then by Umar ibn Khattab as the second caliph, Uthman Ibn Affan as the third caliph, and Ali as the fourth caliph.
