President Donald Trump's decision to end a CIA program that provided weapons and other supplies to rebels fighting the Syrian regime all but certainly amounts to a death knell for the opposition's cause, those familiar with the program say, and an end to the only publicly known U.S. effort to overthrow President Bashar Assad.

"Psychologically, it's a crushing blow to the Syrian rebels," says a former U.S. special operations commander with current experience in Syria, who, like others who spoke to U.S. News for this story, asked to remain anonymous to discuss sensitive information.

The Washington Post reported Wednesday that Trump spiked the five-year-old covert program, which even supporters say failed to achieve its ultimate goal of at least securing a dissident movement in the autocratic Middle East nation, if not precipitating Assad's ouster.

Ultimately, the program spent hundreds of millions of dollars training and dispersing weapons to militia in Syria willing to fight against Assad's regime, according to officials who worked with it. It peaked during its first four years and then gradually tapered off before becoming openly questioned by those carrying out the mission once the rebel stronghold of Aleppo fell last December.

The program, first revealed by The New York Times in June 2012, was born from, and perhaps complicated by a reluctance within the Obama administration to commit the U.S. fully to regime change in Syria in the aftermath of the 2011 Arab Spring movement over concerns of regional chaos. The concern remained even after the U.S. began deploying forces to fight the Islamic State group in Syria after its rise in 2014.

The covert operation also floundered over concerns that so-called "moderate" rebels were operating in coordination with al-Qaida affiliates like Jabhat al-Nusrah either proactively or out of desperation.

It was not without its successes. The militias the U.S. supported achieved some tactical victories, albeit ultimately losing in the face of the Syrian army. The program also developed an intelligence network in Syria that did not previously exist, offering the White House unique perspectives into what was actually happening on the ground through a series of communication tools, including social media platforms like Snapchat.

The highly complicated nature of the war, however, could not comport with the policies the Obama administration set for itself.

"We certainly questioned it all the time, when the direction of the war really started changing, especially when Russia got in, these fighters were pushed to work with the most capable group on the ground, which was Nusrah," a former senior defense official under the Obama administration says.

"These groups were useful in contacting other groups and getting them, even though it strategically didn't survive, tactically at times they were able to get groups to stand down," the official says. "They had better reach than anyone else we were talking to."

Canceling the program, however, was ultimately the right decision, the official says, adding, "the output does not make sense."

A Thursday analysis from the Soufan Group, a New York-based private intelligence firm, noted among the program's shortcomings "the complete inability of the U.S. to adequately vet for violent extremism among the Syrian opposition." The group pointed out how moderates and extremists regularly intermingled and that the moderate rebels were frequently "viewed with suspicion and disdain as tools of the West."

Top members of the Obama administration considered shutting down the program throughout its operation, according to officials who spoke to the Post and others to U.S. News. However, particularly after Russia deployed its military to Syria in 2015 in support of its patron Assad, the White House at that time was concerned about the effect of shutting down the program without having achieved anything from it.

"From a business perspective, it's absolutely the right call," the former commander says. "But the difference between us and the rest of the world, is we have a moral and ethical decision also to make."

Conducting covert operations against another country brings with it "brand" concerns, he says, centered on how the U.S. and its partners are perceived at the time, and how that partnership might affect future operations either for America or for those who choose to partner with it.

"You're worried about not just the brand of the country but the people you're trying to work with, the indigenous folks on the ground you're working with. It's difficult, difficult work."

Other operations on the ground in Syria at the behest of Moscow indicate the Syrian rebels' cause, known through such groups as the Free Syrian Army, is likely doomed.

Iran announced through its state media on Wednesday that Russian officials have orchestrated a second cease-fire around the capital Damascus and the city of Homs – once considered the rebel capital that as of this week has been all but retaken by forces loyal to the Syrian regime. The new agreement signals there are no longer any major outside forces operating in Syria that would support the opposition movement.

Critics of Trump say his cooperation with Russia, including following through on ending the CIA program that Moscow has criticized, amount to playing into Vladimir Putin's hands.

"Making any concession to Russia, absent a broader strategy for Syria, is irresponsible and short-sighted," Sen. John McCain, the chairman of the powerful Armed Services Committee, said in a statement. The Arizona Republican is currently recovering from medical procedures this week that determined he has brain cancer.

Trump has frequently repeated his willingness to work directly with Russia to broker a cease-fire in Syria – a route to peace the Obama administration was hesitant to embrace over concerns of validating what it considered Russia's meddling in Syria and other places, like Ukraine.

However, the CIA program, even in the best-case scenario, was never destined to achieve its mission of removing Assad.