Complex sentence: The latter did not accept Emir Yusuf's request for assistance, viewing al-Jazzar as a reliable representative and whose control of Beirut would both prevent another important Syrian port city from falling to Zahir and provide a launch point for an offensive against Zahir.
Split into simpler sentences: The latter did not accept Emir Yusuf's request for assistance. Al-Wakil viewed al-Jazzar as a reliable representative whose control of Beirut would prevent another important Syrian port city from falling to Zahir and provide a launch point for an offensive against Zahir.