WASHINGTON — The end came quickly for one of the costliest covert action programs in the history of the C.I.A.

During a White House briefing early last month, the C.I.A. director, Mike Pompeo, recommended to President Trump that he shut down a four-year-old effort to arm and train Syrian rebels. The president swiftly ended the program.

The rebel army was by then a shell, hollowed out by more than a year of bombing by Russian planes and confined to ever-shrinking patches of Syria that government troops had not reconquered. Critics in Congress had complained for years about the costs — more than $1 billion over the life of the program — and reports that some of the C.I.A.-supplied weapons had ended up in the hands of a rebel group tied to Al Qaeda further sapped political support for the program.

While critics of Mr. Trump have argued that he ended the program to curry favor with President Vladimir V. Putin of Russia, there were in fact dim views of the effort in both the Trump and Obama White Houses — a rare confluence of opinion on national security policy.

The shuttering of the C.I.A. program, one of the most expensive efforts to arm and train rebels since the agency’s program arming the mujahedeen in Afghanistan during the 1980s, has forced a reckoning over its successes and failures. Opponents say it was foolhardy, expensive and ineffective. Supporters say that it was unnecessarily cautious, and that its achievements were remarkable given that the Obama administration had so many restrictions on it from the start, which they say ultimately ensured its failure.

The program did have periods of success, including in 2015 when rebels using tank-destroying missiles, supplied by the C.I.A. and also Saudi Arabia, routed government forces in northern Syria. But by late 2015 the Russian military offensive in Syria was focusing squarely on the C.I.A.-backed fighters battling Syrian government troops. Many of the fighters were killed, and the fortunes of the rebel army reversed.

Charles Lister, a Syria expert at the Middle East Institute, said he was not surprised that the Trump administration ended the program, which armed and trained thousands of Syrian rebels. (By comparison, a $500 million Pentagon program that envisioned training and equipping 15,000 Syrian rebels over three years, was canceled in 2015 after producing only a few dozen fighters.)

“In many ways, I would put the blame on the Obama administration,” Mr. Lister said of the C.I.A. program. “They never gave it the necessary resources or space to determine the dynamics of the battlefield. They were drip-feeding opposition groups just enough to survive but never enough to become dominant actors.”