President Obama’s decision to begin directly arming Syrian rebels comes nearly two years after he first called for Bashar al-Assad to resign — and at a time when the Syrian leader’s forces and outside friends have turned the war back in his favor.

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, with Russia, Iran and its sponsored militias on the other.

The American policy shift is being dictated by the urgency of the situation on the ground in Syria and the problems that poses for achieving a negotiated end to the civil war on terms that Obama favors.

U.S. officials maintain that Assad cannot remain in power under any agreement that might emerge from a future peace conference in Geneva, a process that Secretary of State John F. Kerry and Russian Foreign Minister Sergei Lavrov endorsed last month.

But with Syrian government forces now winning the war, there is little incentive for Assad to agree to any deal that would require him to give up power. His recent military gains have been propelled by the arrival of Iranian-backed Hezbollah fighters from neighboring Lebanon and a steady supply of Russian weapons.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

“Between now and then, there does need to be a change of things on the ground,” said a Western diplomat, who requested anonymity to discuss internal diplomatic deliberation regarding Syria, referring to future peace talks. “This is an important part of the preparation for Geneva.”

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

On the eve of the summit Thursday, administration officials announced that they have concluded that Assad has used chemical weapons against opposition forces, crossing a “red line” that Obama had drawn previously. French, British and Israeli officials had come to the same conclusion several months ago.

But Russian officials on Friday called the evidence shared by the U.S. administration and its European allies inconclusive, setting up a difficult meeting between Obama and Putin in the days ahead.

“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,” Ben Rhodes, the deputy national security adviser for strategic communications, told reporters Friday. “There are no illusions that that’s going to be easy.”

Western diplomats say Putin is unlikely to soften his support for Assad at the G-8 summit, given Russia’s strategic interest in preserving the Syrian leader in power and in facing down a U.S.-backed policy initiative to oust him.

Alexander Lukashevich, spokesman for the Russian Foreign Ministry, said at a briefing Friday in Moscow that reports about the American decision to send arms to the Syrian opposition “cannot fail to be perceived with serious concerns.”

“They prompt, among other things, the idea that the U.S. effort to ensure the due representation of the opposition at the prepared international conference will not gain any traction,” he said.

More than 90,000 Syrians have been killed in a worsening civil conflict since Obama first declared in August 2011 that Assad had lost the legitimacy to rule.

His caution — supported by a majority of Americans surveyed in recent polls — has angered some congressional Republicans and human rights activists while leaving European allies waiting for a clear sense of the direction he wants to take in Syria.

Most European leaders understand Obama’s domestic political motivation: He was elected as a president who would end America’s wars, not start new ones.

But Obama’s response to Assad will also be gauged by Iran’s leaders to assess how serious he is about stopping their uranium-enrichment program, which U.S. officials and their allies believe will be used to develop a nuclear weapon.

Nile Gardiner, director of the Margaret Thatcher Center for Freedom at the conservative Heritage Foundation, said Obama is “a bit late to the table” in expanding U.S. support to the rebels.

“His stance on Syria has been viewed in some parts of Europe, particularly Britain and France, as very weak and slow,” Gardiner said. “It’s perceived as if Obama had outsourced his foreign policy to the United Nations and Russia.”

After leaving Iraq and setting an end date for American combat operations in Afghanistan, Obama has sought to avoid a new war in a Muslim country, this one at the center of the Arab Middle East.

Obama intervened in Libya two years ago after securing Arab support, based largely on Moammar Gaddafi’s threats to massacre the restive population of Benghazi.

But Obama did so with a very low profile, lending U.S. military aircraft, ship-to-shore missiles and intelligence to a broad effort that featured European leaders as the face of the intervention.

A similar diplomatic approach has taken shape around Syria, although one defined by even more White House caution.

Some analysts said Obama’s decision to confirm Assad’s use of chemical weapons only days before the G-8 summit was designed, in part, to lessen the criticism he was likely to receive from some of his European counterparts.

British Prime Minister David Cameron pushed successfully last month for the European Union to lift its embargo on sending arms to Syria’s rebels, preparing the way legally for the British and French to begin doing so.

But Cameron has not yet decided whether he will send arms, and he has said that he would seek approval from parliament for any proposal to directly arm Syria’s opposition.

Obama administration officials have also not specified what kinds of weapons the government intends to provide Syria’s rebels, a loose confederation of forces, including a number of powerful factions guided by Islamist extremism.

Rhodes, the deputy national security adviser, said earlier this week that “we are aiming to be responsive to the needs of the Syrian opposition.”

To what degree remains unclear. Syrian rebel leaders have sought antitank, antihelicopter and antiaircraft weapons that many European governments are reluctant to supply for fear they will fall into the wrong hands.

“We’re not yet clear on what the policy response will be,” said the Western diplomat, referring to Obama’s response. “The specifics of what, when and how is not yet clear. What we’re all clear about is that we need a political process.”

Will Englund in Moscow contributed to this report.