The war, entering its fifth year next month, has displaced half of the country's population and claimed the lives of an estimated 370,000 people. But a cease-fire planned for Friday, negotiated by the US and Russia, may provide world powers with their best opportunity yet to discuss the viability of a diplomatic solution to the conflict.

That is the hope of Kerry, US President Barack Obama and Russian President Vladimir Putin, who all called for "genuine," "galvanized" peace talks should the cease-fire take hold.

The International Syria Support Group– comprised of nearly 20 nations with vested interests in the war– seeks a negotiated political transition to a national unity government and, ultimately, nationwide elections, in which the diaspora of Syrian refugees dispersed throughout the Levant and Europe will be allowed to cast votes.

But achieving and holding a cease-fire will be a significant achievement in and of itself, with over one hundred rebel groups individually deciding whether or not to accede to the terms of the "cessation of hostilities"– a pause in the fighting shy of the legal prerogatives of a formal truce– by Friday at noon.

According to a joint US-Russian statement released on Monday, parties to the deal must agree to fully implement a UN Security Council resolution passed in December, which lays out a political roadmap for an end to the war; Cease attacks using "any weapons, including rockets, mortars, and anti-tank guided missiles," against one another; Allow access for humanitarian aid to "people in need" nationwide; And use "proportionate" force if the pause in fighting periodically falters.

"Naturally, we urge the maximum number of armed opposition factions to express their support and readiness to participate in the cessation," State Department spokesman John Kirby said. The High Negotiations Committee– tasked with representing the Syrian opposition at the negotiations– is currently meeting in Riyadh. "We believe this arrangement is an important opportunity to stop the violence, facilitate the delivery of humanitarian assistance, and help provide the space necessary for the political process to continue."

Furthermore, militants associated with the al-Nusra Front– an affiliate of al-Qaida in Syria– will still be fair game for targeting. That may complicate enforcement of the agreement, as al-Nusra fighters have blent in with other rebel groups in several key areas, such as Aleppo, once the country's largest city.

Moscow– which has aggressively intervened in the war in support of Syria's embattled president, Bashar Assad– hailed the agreement on Monday as a success of Russian leadership. Its officials say Assad remains the only legitimate ruler of a united Syria, while those opposed to him– including the governments of the US, Turkey, Saudi Arabia and its Gulf allies– consider him the primary agitator of the conflict and a war criminal.

"What we are looking at is a complete military victory by the Assad front, especially Putin, whose intervention made all the difference," James Jeffrey, former US ambassador to Iraq from 2010 to 2012, told The Jerusalem Post. "Obama has no leverage and no interest, and the whole thing is an unmitigated disaster for the US and its regional order."

At a Senate Foreign Relations Committee hearing on Tuesday, Kerry said the US was planning a meeting in Geneva in the coming days to work out "modalities" with Moscow that will ensure they are accurately targeting al-Nusra's people and property.

The US, meanwhile, will continue its targeting of Islamic State in the country's eastern provinces.

Kerry also acknowledged that the deal, formally outlined on Monday, may fail before it starts. Asked by one senator if the plan could amount to a "rope-a-dope" trick by Russia, Kerry said it may indeed.

But this particular diplomatic effort was cast by the secretary as Syria's last and greatest hope for peace. The state itself may irreparably fall apart, Kerry said, if negotiations fail to succeed in the coming months.

"We're going to know in a month or two whether or not this transition process is really serious," Kerry said. "If there's stonewalling," he added, "we will know."

He warned that rebel groups considered moderate by the US would be the first to declare the effort a farce.

"It may be too late to keep it as a whole Syria if we wait much longer," Kerry said. "There is a significant discussion taking place now about Plan B if we don't succeed at the table."

The Obama administration has come under criticism in recent weeks for its failure to mitigate or offset Russia's intervention in the war, which dramatically turned the tide of the conflict in Assad's favor. Kerry addressed that criticism on Friday with two, often opposing messages.

On the one hand, he acknowledged that military power has indeed provided Russia, Assad, and its allies in Iran with hard leverage– newly created "facts on the ground"– at the negotiating table. But he also said the war was unwinnable on the battlefield, and that Russia was slowly coming to the conclusion that Syria's sectarian makeup would ultimately require Assad's exit.

"Putin and the Iranians will stick with Assad in part out of convenience– the 'devil you know'– and in part because his going would be a symbolic victory for the US and its friends," said Jeffrey, who is now with the Washington Institute for Near East Policy. "Neither Putin nor the Iranians want that."

The Senate committee's chairman, Bob Corker of Tennessee, complimented Kerry on his passion and his effort. But he warned that Russia, through its aerial campaign, was seeking to use refugees as weapons of war– not only seeking to scare civilians away from enemy targets, but also working to flood Turkey and the rest of Europe with a burdensome number of additional asylum-seekers.

Corker also questioned Kerry’s allusion to a “Plan B,” which the secretary said he would only outline should the time come when Plan A– a political settlement– fails.

“Russia knows there will be no Plan B,” Corker responded. The Republican senator said the options before the president – including, presumably, a no-fly zone and humanitarian corridor along Syria’s northern border – have been at his disposal for several years.

In a video conference on Tuesday among Obama, Prime Minister David Cameron of the United Kingdom, President Francois Hollande of France, and Chancellor Angela Merkel of Germany, the White House said the leaders together welcomed the coming cease-fire and "reaffirmed their commitment to a political transition in Syria that will bring lasting peace and legitimate governance to the Syrian people."

"The president also discussed with his counterparts steps to alleviate the humanitarian crisis in Syria," the White House added, "and the importance of the NATO mission and coordination between Greece and Turkey to manage the flow of migrants into Europe."