Complex sentence: The Franks and Mongols also shared a common enemy in the Muslims, but despite many messages, gifts, and emissaries over the course of several decades, the often-proposed alliance never came to fruition.
Split into simpler sentences: The Franks and Mongols also shared a common enemy in the Muslims. However, despite many messages, gifts, and emissaries over the course of several decades, the often-proposed alliance never came to fruition.