What weaknesses [plural] were common to all of the Muslim empires?

This is true, not having a navy has shown itself to be a weakness of many nations/civilizations in the past. And there are also good reasons why this is so. Not having a navy strongly impacts a civilizations ability to maneuver around certain points and go around them. It also limits the ability to trade with other natons. Not to mention the additional value of having the firepower of ships (those that had cannons later on at least).  

Explanation:

All three Muslim empires had poor leadership, and this reason led to all of their downfalls. Their economies crumbled and they could not support themselves anymore. The Muslim empires were very strong starting off, but they could not survive over time.

Explanation:

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