The Surrounded Kurds at Kobani and Afrin
The IS army has been battling for Kobani for over a month now. Their epic advance and sweep that brought them there with up to 4000 men in late September seemed to guarantee a victory, as the Kurds only fielded perhaps 1500 to hold the city. But IS failed to seize it despite it coming close to succeeding because of three factors:
The defiance of the Kurds (both men and women), a battle in an urban environment, and US airpower. The great equalizer was the American airpower. While not enough to insure a victory, the scales were tipped to stall the IS advance. The urban environment was one that neither side really knew much about or had experience with. It proved much harder than expected for the attacker than Kobani's defenders. Urban warfare is even difficult for the most trained and experience soldiers, and the Kurds and IS were guerrilla styled warfare. While the Kurdish defenses nearly collapsed in early October when much of the town was in IS hands, it held on until another 150 men arrived to balance the scales. The Islamic State's army has suffered much more than they expected, up to 600 of their men killed there. Their bodies are just rotting. The airstrikes forced them to use different strategies making it harder to move around and mass in force. Their own strategy started to divert resources to other areas that were easier to win, such as Anbar, where 80% is theirs and has forced a doubling of American combat advisers around Baghdad. IS is finding out that while American airpower cannot halt them seize ground, it makes it much more expensive and it becomes much more difficult to so so.
About 120 miles west of Kobani is Afrin. Another city of one million in Kurdish held areas. The defenders there are just waiting for the war to begin. For there, they are surrounded not by IS troops, but an equally brutal and well armed army, al-Qaeda's Nusra. In case your memory has lapsed, during the first two years of that war, it was the Nusra group that was the spearhead of the Free Syrian Army. They were the men, so well armed and supported by al-Qaeda that seized Aleppo, Homs, cities that were FAR larger than Kobani. Nusra was akin to the SS troops in WW2. Without them, the FSA would have had no success.
Since then, Nusra and the FSA have become enemies, with Nusra becoming the real power opposing Assad. Afrin is preparing for it to be the next Kobani. Nusra has made a pact with the neighboring sects in the area that oppose the Kurds. Like the Kurds at Kobani, they are defiant with the small arms they have, but Afrin will be much harder to control because of its size. The Kurds only ally on the ground in Syria is the FSA. The FSA has send troops to Kobani and have them in Afrin, as well.
The FSA is supported by the Arabs, namely, UAE, Saudi Arabia, Jordan. They have been sending them supplies for over a year.