Aleppo is one of the crown jewels of the ancient world, Syria's largest, richest and -- for the past three months -- most dangerous city. The battle that is raging there has reduced entire neighborhoods to smoking piles of rubble.

Across Syria, hundreds of lightly armed, poorly trained rebel groups are fighting to overthrow one of the Middle East's last remaining dictators, Bashar al-Assad, who is using the might of the Syrian military to crush the uprising and hold on to power.

As many as 30,000 Syrians have been killed. More than two million have been forced out of their homes. In Aleppo, tired rebels are being reinforced by foreign fighters many of whom are radical Islamists.