by Azmat Khan

August was the deadliest month since the Syrian rebellion began a year-and a-half ago — and more deadly for civilians than the bloodiest months in the Iraq and Afghanistan wars, as well as the recent uprisings in Tunisia, Egypt, Libya, Yemen and Bahrain.

More than 5,400 people, a significant majority of them civilians, were killed during the month, according to the Syrian Observatory for Human Rights, a widely cited London-based opposition group that tracks the violence*. The group puts the total number of both civilian and military deaths since the uprising began at 26,000, which suggests that about 20 percent of recorded deaths occurred in August alone, when the regime began unleashing more air power to crush the revolt and fighting reached the country’s largest city and its commercial capital, Aleppo.

But these numbers, as stark as they are, cannot fully illustrate the escalating war playing out on the ground. In The Battle for Syria, airing on Tuesday, FRONTLINE journeys into the heart of the rebellion in Aleppo for an up-close, unflinching look at how the insurgency is operating, why the rebels are fighting and where the war is leading.

Guardian correspondent Ghaith Abdul-Ahad, reporting for FRONTLINE, accompanies rebel commanders who are waging a full-scale assault on President Bashar al-Assad’s army while trying to overcome internal rivalries between secular and Islamist fighters.

The conflict has spilled into the region, with an estimated 200,000 refugees fleeing to neighboring states, including Turkey, Lebanon, Jordan and Iraq. U.N.-Arab League envoy Lakhdar Brahimi is on his first mission to Syria today, where he will be meeting with the embattled president.