A multi-day siege on the key Tabqa Air Base in Syria’s northern Raqqa Province has ended with an ISIS victory, and the ouster of the Syrian military from their last possession in the province.

That means ISIS now controls the entirety of Deir Ezzor and Raqqa Province, and also holds portions of Hasakah, Aleppo, Hama, and Homs. In addition, of course, ISIS controls a considerable portion of Iraq, including virtually the whole Anbar Province.

The battle began poorly for ISIS, with Syria’s military knowing the offensive was coming, ambushing the fighters en route and killing 150. The battle continued through the weekend, and ultimately over 500 were killed, including 170 troops and 346 ISIS fighters.

That ISIS lost twice as many troops and still won the battle reflects how their ability to muster huge amounts of fighters and secure thousands of new recruits monthly is enabling them to expand into even fiercely defended territory. The air base was more or less out of supply for Syria, without the ability to get more troops into the area easily to defend the site.