WASHINGTON (Reuters) - President Obama's decision to arm Syrian rebels for the first time follows an intense, nearly two-year debate within the White House in which the president and his closest advisers consistently expressed skepticism about U.S. intervention in a Middle East civil war, current and former officials said.
The two deciding factors in the decision to change course, they said, were growing military gains by forces loyal to Syrian President Bashar al-Assad, aided by the Iranian-backed Hezbollah militia, and harder intelligence that the Syrian military had used chemical weapons in the form of sarin nerve gas.
Which of the developments played a greater role in tipping the balance was unclear. Publicly, the Obama administration pointed to the evidence of chemical weapons use, which one senior administration official said had "ripened" in the last two weeks.
Some of the U.S. officials said on Friday that the real game-changer in Obama's calculus on Syria was not the chemical weapons issue - which had been known about for months - but the growing role of Lebanese-based Hezbollah.
The battlefield advances by Hezbollah have raised the prospect that Assad could stay in power for some time. The Shiite militia's decision to get more directly involved in the conflict against mostly Sunni rebels has also heightened the war's sectarian divide, and increased Sunni-Shiite tensions in neighboring Lebanon.
U.S. officials and European diplomats also cited as a factor in Obama's decision a looming meeting next week with G8 allies - especially France and Britain - in which Syria will be a major issue.
"Had they not made the beginnings of a move on the issue, the G8 meeting would have been pretty hard on the president," said a European diplomat, speaking on condition of anonymity.
Obama was roundly criticized by Syria hawks in the U.S. Congress and elsewhere for first suggesting in April that chemical weapons may have been used in the Syrian civil war, crossing a "red line" he set last year, but not then following up with actions against the Damascus government.
But in an interview 10 days ago, well before the White House announced Obama's decision on Thursday, a senior aide insisted that the chemical weapons question had never been dropped.
There are "clear instructions from the president that we are not walking back from the red line," the senior administration official said at the time. "The intelligence community is all over this."
The White House on Thursday announced that it had concluded that Syrian forces had used chemical weapons, and said Obama had decided to supply direct military assistance to the opposition.
While Obama crossed a line of his own in making the move - the White House had for more than a year resisted calls to arm the rebels - he appears determined to keep the United States from getting sucked too deeply into Syria's sectarian civil war.
European officials and others with close knowledge of the situation said the United States would supply the Western-backed Syrian Military Council with automatic weapons, light mortars and rocket-propelled grenades.
While significant, the weaponry will not include shoulder-fired surface-to-air missiles known as MANPADs that could bring down Syrian military planes and helicopters, officials said.
And for now, Washington is not backing the establishment of a "no-fly zone" over Syria, which would involve a major commitment of U.S. and European air power to counter Syria's extensive air defenses, they said, in part because there is no international consensus on the step.
"This is, in a way, a low-cost option," a former U.S. official with extensive contacts in the region said of the White House's new steps, worrying that the U.S. military aid was months too late.
The White House and State Department declined to publicly detail what sorts of weaponry and other materiel will be sent to the rebels, or how quickly it will arrive.
While Obama has been consistently cautious about U.S. involvement in Syria, his team has been at times less than unified.
Obama's original decision, in August 2011, to call on Assad to leave power, was preceded by intense debate in Washington, London and other capitals, according to diplomats and former officials.
The Pentagon has been consistent in opposing deep U.S. military involvement, such as a no-fly zone.
Last fall, however, then Secretary of State Hillary Clinton and CIA chief David Petraeus presented a joint proposal for the United States to arm the rebels. The White House turned down the idea.
In the current situation, Secretary of State John Kerry is said to have been active in pressing Obama on the need to do more.
"The constituency within the administration for doing more is much more significant that it was in the past," said Dennis Ross, who was a senior Middle East adviser to Obama. "But the hesitancy remains, I believe."
Ross, who left government in December 2011, said that during his time at the White House, Obama would closely question the wisdom and consequences of Syria options presented by his advisers.
"What's fair to say, he wanted to be very cautious about the kinds of commitments we would make, he wanted to know even then, look if we're going to take steps, I want to hear, tell me where that leads to," Ross said in an interview this week. "Tell me what's the consequence of doing 'X'. And tell me how that is going to improve the situation and make an outcome that we favor more likely."
Ross said that during his time at the Obama White House, the U.S. experience in recent wars, which highlighted the difficulty in changing conditions in Islamic countries, weighed heavily in the president's thinking.
"I don't think you can look at it independently from Iraq and Afghanistan. And particularly the sense that these are easy to get into and hard to get out of," Ross said.
Obama's calculus would have been different, he added, if there had been a "more coherent, more credible and more compelling" opposition in Syria.
A senior Western diplomatic source gave a similar account, saying Obama essentially tells his aides to prove to him that American intervention would improve the situation.
"It's a legitimate position," this source said. "I don't see at this point the Americans authorizing the delivery of heavy weapons."
It remains to be seen whether the aid will change a military picture that has seen Assad's forces, backed by Hezbollah fighters, steadily regain ground against the rebels, capturing the key city of Qusair and preparing for an assault on rebel-held areas of northern Syria.
"We believe that we can make a difference," White House deputy national security adviser Ben Rhodes said, when asked if the U.S. aid was too little. He noted that Arab nations and Turkey are also supporting the Syrian opposition.
State Department spokeswoman Jen Psaki said Obama's decision was made some days ago, but would not be more specific.
"This has been something that the national security team and the president has been discussing for weeks. I know the White House said the president decided long before this week," Psaki said.
Additional reporting by Lesley Wrouhgton, Mark Hosenball, Susan Cornwell in Washington, Lou Charbonneau in New York and John Irish in Paris; Editing by Alistair Bell and Tim Dobbyn