Why were nomadic groups like the parthians able to conquer civilizations like the seleucids?

The Parthian empire was a major Iranian political and cultural power in ancient Iran and Iraq. They were able to conquer civilizations like the Seleucid as they were able to seize Media and Mesopotamia from the Seleucid making them lose control of their eastern land and it was replaced by the Romans.

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explanation:

although the crusades failed to capture jerusalem, they had several major impacts on western europe. they increased the authority of the king: sometimes nobles died in battle without leaving an heir in which case the king got their land. kings passed taxes to pay for the crusades.