On Tuesday, Iran’s foreign minister, Mohammad Javad Zarif, charged that the United States had helped destabilize the region, and portrayed Iran as merely defending its interests. Washington, instead, views Iran’s aid to the Assad government as part of an effort to restore itself as a major regional power.
From the start, there were doubts that arming disorganized, often internally fractious forces would succeed. Officials in the Obama administration conceded that there was no way to predict the future loyalties of those who received American arms, despite a lengthy vetting process. That problem — getting the weapons into the right hands and assuring they were not passed on to others and used against American troops or allies — plagued the effort soon after it was proposed by Hillary Clinton, who was then secretary of state, and David H. Petraeus, the C.I.A. director at the time.
Mr. Trump’s decision was first reported by The Washington Post. But it was foreshadowed as early as April, when the Trump administration said that ousting Mr. Assad, whose government has fought a civil war that has taken roughly half a million lives, was no longer a priority. Instead, the United States and Russia have been discussing cease-fire zones in the country, the first of which went into effect this month.
Those discussions have been possible because Mr. Assad, secure in his support from Moscow and Tehran, no longer sees a fundamental threat to his ability to remain in power. And Mr. Trump’s decisions amounted to an acknowledgment that no escalation of the program, which began in 2013 in concert with the C.I.A.’s counterparts in Saudi Arabia, Turkey and Jordan, was likely to yield a different result.
When it began, the initial objective was to force Mr. Assad to the bargaining table, in a series of negotiations that the secretary of state at the time, John Kerry, took up in earnest in late 2015. But each agreement — for cease-fires, and deadlines for a political “road map” for elections in the country — fizzled. Mr. Kerry fumed that Mr. Obama was not willing to provide the kind of military pressure on Mr. Assad that might bolster the diplomacy. Mr. Obama, for his part, was leery of entering another Middle East war whose outcome he could neither control nor predict.
The program became less relevant as the Russians increased their presence in Syria, targeting and badly weakening the C.I.A.-backed rebels, who were the most capable of the opposition fighters. That helped the Assad government claw back and consolidate territorial gains.
“This is a big deal, but it’s been a long time coming,” Charles Lister, a Syria analyst for the Middle East Institute in Washington, said. “It’s the biggest indication so far of the administration’s having given up on the opposition.”
“After all, the Southern Front has consistently been our most reliable anti-Assad partner,” Mr. Lister said, referring to opposition forces fighting Mr. Assad in the southern part of the country. “It’s also the result of strong Jordanian pressure, as Amman has been pushing a freeze for a long time. So it was probably inevitable, but it’s nonetheless very significant.’’
He added that it was “a big mistake in my mind.”
Other independent experts said it was unclear whether Mr. Trump’s decision would have an impact on fighters defending areas held by the opposition.
At its height, the program was run through operations rooms in Jordan and Turkey, supporting rebel groups fighting under the banner of the Free Syrian Army who were deemed not to be extremists.
But the pressure on Mr. Assad was not great enough to force him to enter negotiations to end the civil war. Nor was it sufficient to clear the way for the rebel groups to take over major cities or approach the capital, Damascus. The program also sought to bolster so-called moderate rebels against extremist factions like the Syrian affiliate of Al Qaeda.
When the history of the effort is written — and the documents surrounding it are declassified — historians will doubtless seek to learn why the rebels lost ground for years, to Syrian government forces and their Russian and Iranian allies, and to extremists.
After the rebels’ expulsion from the eastern half of the city of Aleppo last year, it became clear that they no longer posed a serious threat to Mr. Assad’s rule.
But stopping the covert program, which mainly helped rebels near the Turkish border in northwestern Syrian and along the Jordanian border in the south, will not affect the fight against the jihadists of the Islamic State in the east. A different program there run by the Pentagon is supporting a Kurdish-Arab militia known as the Syrian Democratic Forces.