President Obama’s decision to begin arming the Syrian rebels followed more than a year of internal debate over whether it was worth the dual risks of involving the United States in another war and seeing U.S. weapons fall into the hands of extremist groups among the rebels.

The White House said the final push came this week after U.S. intelligence agencies concluded with “high certainty” that Syrian President Bashar al-Assad’s forces had used chemical weapons against the rebels.

But U.S. officials said that the determination to send weapons had been made weeks ago and that the chemical weapons finding provided fresh justification to act.

As Syrian government ­forces, with the help of Hezbollah and Iranian militias, began to turn the war in Assad’s favor after rebel gains during the winter, Obama ordered officials in late April to begin planning what weaponry to send and how to deliver it.

That decision effectively ended the lengthy disagreement among those in the White House — primarily Obama’s political advisers — who argued that providing arms would be a slippery slope to greater involvement, military leaders who said it would be too risky and expensive, and State Department officials who insisted that Syria and the region would collapse in chaos if action were not taken, officials said.

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

Even after Thursday’s announcement, critics in Washington, rebel leaders and even some U.S. allies described the prospect of sending light arms and ammunition as disappointing. The rebels have asked for armor-piercing and anti-aircraft weapons as well as other heavy equipment.

The administration has continued to deflect questions about what equipment it will provide. Any use of the U.S. military “would be things we could discuss in great detail,” Benjamin J. Rhodes, Obama’s deputy national security adviser for strategic communications, said Friday. But “when you get into questions of provision of assistance to opposition groups, we are just more limited in our ability to say, well, here is a list.”

In taking a modest first step onto Syria’s battlefield, Obama is joining a proxy war far more complicated than it was even a few months ago. It now features the United States and its European and Arab allies on one side, and Russia, Iran and its sponsored militias on the other in support of Assad.

The rapidly shifting balance in the war has made untenable the peace talks proposed last month by Secretary of State John F. Kerry and his Russian counterpart, Sergei Lavrov. With the opposition in a position of weakness, and little immediate incentive for Assad to agree to any deal requiring him to give up power, talks initially scheduled for late May are now unlikely to take place before fall, according to officials and diplomats who spoke on the condition of anonymity to discuss internal administration deliberations.

“There does need to be a change of things on the ground,” said a Western diplomat.

The topic will be taken up by leaders of the Group of Eight, who convene Monday in Northern Ireland. Obama will also meet with Russian President Vladi­mir Putin on the summit’s sidelines to discuss Syria, among other issues.

On Friday, Russian officials called the evidence of chemical weapons use shared by the administration and its European allies inconclusive, setting up a potentially difficult Obama-Putin exchange.

“We still continue to discuss with the Russians whether there’s a way to bring together elements of the regime and the opposition to achieve a political settlement,” Rhodes said. “There are no illusions that that’s going to be easy.”

Western diplomats and U.S. officials said Putin is unlikely to soften his support for Assad at the summit, given his belief that time — for the moment at least — appears to be on the Syrian leader’s side.

More than 90,000 civilians have been killed in the worsening civil conflict, now in its third year. Obama’s caution — supported by a majority of Americans surveyed in recent polls — has angered some congressional Republicans and human rights activists, while leaving European and regional allies frustrated by what they see as U.S. dithering.

“What we’ve asked for is not weapons but U.S. leadership,” said one senior Arab official whose government helps fund the rebels.

Governments in the region, aside from Iran, are virtually uniform in their desire to see Assad go. But they have been divided over what they would like to see replace him. At an April meeting with core opposition supporters in Istanbul, Kerry forged an agreement that all military aid — including from Persian Gulf countries that the United States believes has provided weapons and financial support to Islamist extremist groups among the rebels — would come together behind the U.S.-backed Free Syrian Army’s Supreme Military Council, headed by Gen. Salim Idriss.

That effort has had mixed results. Saudi Arabia, the United Arab Emirates and Jordan all believe that Qatar and, to some extent, Turkey continue to allow money to be funneled to extremists, although in reduced amounts. U.S. intelligence estimates that extremists constitute less than 10 percent of a rebel force of about 70,000, but they have been particularly effective on the battlefield.

U.S. officials believe that the change in policy will help project leadership and coalesce their international backers. It will also, they said, provide a psychological boost to rebel ­forces spooked by what officials described as an exaggerated view of the strength of Assad’s ­forces and their Hezbollah allies. An outright rebel win is seen as both unlikely and less desirable than a negotiated settlement that leaves Syrian institutions intact.

Divisions within the Obama administration on Syria date at least from last summer, when then-Secretary of State Hillary Rodham Clinton and then-CIA Director David H. Petraeus advocated a limit plan to provide arms. The Pentagon has been consistently leery of U.S. involvement, arguing that true military options such as a no-fly zone or even the use of standoff weapons to degrade Assad’s air assets would inevitably draw the United States into direct confrontation. Others were concerned that U.S. weapons would end up in the hands of al-Qaeda-linked extremists.

Kerry, despite palpable frustration in recent months over opposition disorganization, has said arming the rebels is likely to have a “multiplier effect” among other nations supporting them. He called several key foreign diplomats Friday to promote or defend the new U.S. policy, including Lavrov and his British, French and Turkish counterparts.

Previous promises of ­non-lethal supplies from the United States have been slow to materialize on the ground. Congress was just notified this week that the administration intended to provide $123 million in body armor, night-vision goggles and other supplies to Idriss’s Supreme Military Council, aid that Kerry announced at the end of April.

“It’s true that there have been times in which we couldn’t move assistance quite as fast as we would have liked,” Rhodes said Friday. But, he said, there have been marked improvements over the past several months in both communications and transportation pipelines to the rebels and among them. He said the administration was “confident” the new aid would be delivered “in a relatively timely manner.”

Despite reports that the administration is also seriously considering implementing a no-fly zone over some rebel-held areas, Rhodes said, “we haven’t ruled out options, but I think people need to understand . . . the difficulty of some of the options that have been presented.”

At this point, he said, a no-fly zone was not seen by the administration as being in U.S. national interests.

He also said the United States and its allies would not move to destroy Assad’s chemical weapons stocks. “These are dangerous weapons, and the notion that you can destroy them if you aren’t physically present is an extremely challenging one,” he said. “The preference would be to have this be a priority for the international community . . . in post-Assad Syria.”