Hoo boy, is this a complicated question. I took a semester long seminar on Ba’athism in history grad school and the best text written on the topic is Asad: The Struggle for the Middle East by Patrick Seale. I recommend reading it, as well as Michel Aflaq’s admittedly biased books about the Party as primary sources, as my synopsis will do the issue little justice

Ba’athism had three main founders. Zaki Al-Arsuzi was an Alawite, Michel Aflaq was an Arab Christian, and Salah Al-Din Al-Bitar was a Sunni Muslim. Al-Arsuzi founded the original Arab Ba’ath party in late 1940. It was not a success. At about the same time, Aflaq and Al-Bitar, who had studied together in Paris (at this time Syria was still a French colony), created a similar party that they renamed the Arab Ba’ath Party in 1943. Al-Arsuzi was a rather uninspiring figure in the early days, and he lost most of his followers to Aflaq’s movement, which ultimately took over the new Syrian state after independence.

However, now in control of the Syrian Ba’ath, Aflaq overstepped his bounds in his attempts to bring about pan-Arabism. Reaching out to Egyptian leader Nasser in the late 50s, a plan was formed to establish a unified state—the United Arab Republic. However, Nasser disliked the Ba’ath party. Ultimately Aflaq sacrificed the party by acquiescing to unification and de facto Egyptian leadership of the combined new country. Many Ba’ath leaders, including Al-Bitar, viewed this as a betrayal. Al-Bitar and others formulated a coup against the Republic in 1963, and in 1966 Aflaq was exiled from Syria (Al-Bitar also ironically ended up fleeing into exile where he would live until he was assassinated by Asad thugs in 1980)

Well, where did Aflaq go? To Iraq. See, the British-backed King of Iraq was overthrown and executed by his own military in 1958. A series of military officers ran the country, but the Ba’ath party grew in prominence there as a cultural movement led by two men: Al-Bakr (the leader) and Saddam Hussein (the power behind the throne). Al-Bakr and Hussein gladly accepted Aflaq to provide their movement legitimacy. They would take over Iraq entirely just two years later.

In the meantime, seeking legitimacy amid the strife, the coup leaders in Syria welcomed Al-Arsuzi back into the fold and proclaimed him as the “true leader” and spiritual/intellectual force behind Ba’athism.

Ultimately, the naked power struggle between Saddam and the Asads eclipsed the founders of Ba’athism and even the ideology on which it was based. Yet even into the late 80s, Saddam still gave Michel Aflaq a figurehead position and proclaimed him as the “Secretary-General” of the party.