Every human society is dedicated to Law. Okay, maybe not dedicated...in a fantasy setting, some might actively dedicate themselves to Chaos, but even they don't go full Chaotic. There's always some amount of Law in a mortal society. In the real world, though, I stand by my original statement.
Now, it's not necessarily the same Law that they're dedicated to. Those heathens to the east of our homeland worship the wrong gods, and hold the wrong colors holy. (Meanwhile, in the east, they're complaining that the savages to the west don't even know how to propitiate the rain gods and that's why they live in a damn desert. Also their numerology is heretical at best, if not outright blasphemous.) But every real world society (and even over 90% of fantastic societies) have ideas about who the gods are, what makes them happy, who you can marry, what you can eat, how you should dress, how different segments of society should interact with each other, et cetera, ad infinitum, and most of them are not very chill about it when one of these rules/customs/laws is broken.
And when two different Laws collide, whether it's religious Law, political Law, economic Law, or sometimes even simple customary Law, you get strife. Wars, border raids, clan feuds, gang violence, any kind of us-vs-them you can imagine. And while a horde of poor Chaotic saps would get ground down pretty quickly and easily (without overwhelming numbers), these are powers of Law we're talking about here. These guys are going to take their chains of command, and supply lines, and strategically placed fortifications, and they're going to enter into a long, bloody war of attrition, and fight until one side is unable to continue, unless both sides wear down at about the same rate and the whole thing just peters out.
Chaos can't exist where there is effective Law. But in all those in-between areas where one Law ends and another begins, Chaos lurks. It's just like magic that way...lurking in the in-between areas, like midnight (not one day nor the next), like Samhain (not one year, but not yet the next). Chaos lurks in between Laws, and when those Laws go to war, Chaos thrives.