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Thousands of rebel and regime troops have gathered in and around the northern Syrian city of Aleppo and many more are on their way, setting the stage for what one rebel commander said would be the "mother of all battles."

According to CNN, 18 of the 22 rebel military brigades are currently stationed in and around Aleppo and another 300 troops are on their way. Meanwhile, the Syrian Army has been steadily withdrawing troops and tanks from other towns and villages and dispatching them to Aleppo – a city that, until last week, some expected to remain out of the fray of the Syrian civil war. Free Syrian Army representative Abu Omar al-Halabi told Deutsche Presse-Agentur that since yesterday, 3,000 rebel fighters have joined the 2,500 already in and around Aleppo.

But, according to CNN, 48 of yesterday's 200 deaths across the country happened in this city better known for being a UNESCO World Heritage site. This was the first time Aleppo led the country in death toll, marking it as this week's ground zero.

The United Nations' human rights agency warned today of an "imminent" showdown, according to the Associated Press, while US State Department spokeswoman Victoria Nuland told Washington reporters that the US was concerned it would see a "massacre" in Aleppo. "That's what the regime appears to be lining up for," she said, according to The New York Times.

"The stakes could not be higher," writes BBC's Ian Pannell, who has been reporting from inside Aleppo. "For the armed opposition losing would be a disastrous setback that, at the very least, could neuter their revolution for months. For President Bashar al-Assad, losing Aleppo could be the tipping point that presages the downfall of his government."

Describing a scene of "blackened carcasses of tanks and military supply vehicles" in the city streets, Mr. Pannell writes:

The debris of rubble and twisted metal parts on the road marks a small defeat for the army and a gross miscalculation by local military commanders as to the strength of the rebellion they face in the city. Perhaps with good cause. With the exception of peaceful protests by students at the university and others in some outlying districts, Aleppo has seen hardly any of the fighting and deaths that have occurred across Syria. That all changed in the last six days as clashes erupted in a handful of districts, quickly spreading to others. The Free Syrian Army claims to control 70 percent of neighbourhoods in Aleppo.

Government troops were "simply unprepared" for the rebels' strength when they first attempted to retake the city earlier in the week – but now that they have had time to regroup, "neighborhoods are being pounded by artillery, mortars, and helicopter gunfire," Pannell writes. Now, "Despite their confidence and commitment, the rebels remain vastly outgunned and, with reinforcements from the army, outmanned, and it is hard to see how they can prevail."

Describing the state of play in the city to Reuters, opposition activist Majed al-Nour said that the rebels "have a foothold" in parts of central Aleppo, as well as the east and west of the city. But regime troops control the entry points, as well as the main roads and commercial streets, and they are waging a campaign to take back residential neighborhoods held by the rebels.

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Time magazine's Tony Karon warns that despite the stunning rebel takeover of parts of Damascus and Aleppo, the change in which group has the upper hand will be brief. The Syrian Army was caught off guard, but has now retrenched, and the way it "brutally" pushed the rebels back out of much of Damascus may be repeated in Aleppo.

Comparing today's balance of power in Syria to the Vietcong's Tet Offensive in 1968, which "convinced Americans the Vietnam War was unwinnable," Mr. Karon writes that rebel gains in recent weeks have proven to Syrians that "the regime will not be able to restore its grip over all of the country, or to crush the rebellion by force. For many Syrians, that signals the inevitability of a change of regime — a realization that will convince many of Assad’s less committed allies to switch sides or seek alternatives."