The party has suspended its activities in Syria after Islamist-led rebel forces toppled Assad's government last week, 20 years after its rival twin branch in Iraq was banned, marking the final collapse of a movement that once held sweeping power in both countries.
With Assad gone, "the Baath in Syria... is bound to fully decline," said Nikolaos van Dam, an expert on the party and author of a book about its history, "The Struggle for Power in Syria".
Van Dam said he does not believe "they will ever have an opportunity for a comeback".
The Arab Socialist Baath Party, officially, was founded in Damascus on April 7, 1947, seeking to merge socialist ideals and Arab nationalism.
In its early years, the party recognised the important cultural role of religion for Muslims, who make up the majority in most Middle Eastern countries, while advocating a secular state that could unify the fragmented Arab world across sectarian divides.
But in both Syria and Iraq, whose populations are multi-ethnic and multi-sectarian, the Baath party had become a vehicle for minority rule.
In Iraq, Sunni Muslims ruled over a Shiite majority, while Alawites -- the Assad family -- ruled over Syria's Sunni majority.
Sami Moubayed, a Damascus-based historian and writer, said that both the Iraqi and Syrian branches failed to live up to their slogan of "Unity, Freedom and Socialism".
"There was never unity, let alone freedom," he said.
"Their socialism amounted to disastrous nationalisations," added Moubayed, author of "The Makers of Modern Syria: The Rise and Fall of Syrian Democracy 1918-1958".
- Diminishing 'appeal' -
The Baath had evolved into authoritarianism under Saddam Hussein in Iraq and Hafez al-Assad, and later his son Bashar, in Syria.
"Arab nationalism, particularly secular Arab nationalism, has lost much of its appeal... and thereby also the role of the Baath Party as an Arab nationalist party," said van Dam.
"State nationalism has gradually become more important than pan-Arab nationalism".
In Syria, a military junta dominated by Alawite, Druze and Christian officers seized power in 1963, adopting Marxist-inspired policies.
The party's founders, Michel Aflaq, a Christian, and Saleh Bitar, a Sunni, were sidelined and then fled to Iraq.
Hafez al-Assad, an air force commander, emerged as the dominant figure in 1970, consolidating control over the party and leading Syria in a reign marked brutal repression.
In 2000, his son Bashar took power.
In neighbouring Iraq, the Baath party solidified its grip in 1968 through a military coup led by General Ahmed Hassan al-Bakr.
In 1970, Saddam Hussein assumed control, ruling with an iron fist until his overthrow by a US-led coalition in 2003.
"Both parties only led their countries to failure," said Moubayed.
Under the Baath rule, Syria's military lost territory to Israel in a 1967 war and suffered painful blows in another conflict six years later.
The Iraqi Baath party failed against Iran in the 1980-1988 war, initiated an invasion of Kuwait in 1990, and collapsed under the US-led coalition intervention in 2003.
Despite their shared Baathist roots, the Syrian and Iraqi branches were bitter rivals.
Syria supported Iran during its war with Iraq in the 1980s, reflecting a persistent sectarian divide as Hafez al-Assad aligned with Tehran's Shiite leadership, sidelining Sunni Saddam.
Yet both Baath regimes relied on similar methods of coercion against their domestic opponents.
And both shared another striking similarity.
"The Baathist rulers of both Iraq and Syria became the party," said van Dam.
The parties had their own institutions, "in Iraq better organised than in Syria, but they were fully subservient to their respective presidents," he said.
Moubayed said that although the Baath's decline was inevitable, that may not be the case for the ideals the party had claimed to champion.
"There may one day be a revival of Arab nationalism," he said.
"But it is certain that it will not come from the Baath."