What was the creed of the historian Imam Al-Waaqidi, can the book Futooh Ash-Shaam be ascribed to him, and is this book reliable?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

We could not find any scholarly view about the creed of Al-Waaqidi in the biographies of Islamic figures except for Al-Fihrist by Ibn An-Nadeem, in which he stated that Al-Waaqidi was a Shiite. Ibn An-Nadeem wrote about him, “Abu ʻAbdullaah Muhammad ibn ʻUmar Al-Waaqidi was the freed slave of Sahm ibn Aslam from the tribe of Aslam, and he was a devoted Shiite and faithful adherent to the Shiite doctrine of Taqiyyah (i.e. concealment which amounts to lies to be a religious rite by which one claims to seek the favor of Allaah).” [Al-Fihrist p. 127]

The rest of the books did not make mention of his affiliation to the Shiites and stated only his weakness as a hadeeth reporter according to the scholars of hadeeth. Siyar Aʻlaam An-Nubalaa' reads, “Al-Waaqidi Al-Madeeni Al-Qaadhi, the author of the books and writings on Maghaazi (military campaigns). He is the well-versed scholar and Imaam, Abu ʻAbdullaah, one of the knowledgeable scholars, despite his weakness as a hadeeth reporter, as agreed-upon by the scholars.”

As for your question whether the book Futooh Ash-Shaam was attributed to Al-Waaqidi and whether it is one of the reliable books in this discipline, it has been stated in the books of many scholars that Futooh Ash-Shaam was written by Al-Waaqidi. Az-Zaylaʻi wrote, “Another hadeeth in this regard is that reported by Al-Waaqidi in his book Futooh Ash-Shaam; it reads: 'Ruwaym ibn ʻAamir narrated on the authority of Saʻeed ibn ʻAasim ...'” [Nasb Ar-Raayah, 2: 314]

Moreover, Ibn ʻAsaakir said in his book Tareekh Dimashq, “...as mentioned by Muhammad ibn 'Umar Al-Waaqidi in Futooh Ash-Shaam.”

Al-Hamawy attributed Futooh Ash-Shaam to Al-Waaqidi in his book Muʻjam Al-Udabaa'.

Some scholars underlined that Al-Waaqidi was a reliable historian. Ath-Thahabi wrote, “It is established that Al-Waaqidi is weak (in the field of hadeeth). He is a good scholar in history and military campaigns. We cite his reports without relying on them as evidence.” [Siyar Aʻlaam An-Nubalaa']

Nevertheless, some scholars asserted that most of the contents of Futooh Ash-Shaam cannot be authentically attributed to Al-Waaqidi.” Az-Zirikli wrote, “Futooh Ash-Shaam is attributed to him (Al-Waaqidi), but most of it cannot be authentically attributed to him...” [Al-Aʻlaam]

Allaah knows best.