TLDR Up Front: ISIS has expanded its territorial reach declaring a Willayat in the chaotic ungoverned areas of Libya. But it’s not entirely clear the militias there will be as easy to displace as in Syria and it is very unlikely the world will ignore them long enough to gain a foothold.
New neighbors are the worst.
Full Context in the Back: ISIS claiming Sirte and its surrounding areas as territory in Libya means two things. The first they now hold territory, that they have proclaimed the Willayat Braqa and rule a population (est. ~80,000) which may kick off the same kind of growth cycle dynamics that occurred when Ar-Raqqa was taken in 2013 and Fallujah in 2014. Within the territory around Sirte they have also captured a few (mostly destroyed) military bases, a power plant…the militia forces have retreated to a perimeter around Misrata. There are also significant oil assets in the area but it’s not clear whether the Islamic State has taken control of them.
The second. I can now recycle all those bad puns I used in reference to Libya in 2011.