But I thought they were on my side...

[T]he stars are powerful agents in and of themselves who, when called for, are willing to do Yahweh’s martial bidding. They are active, conscious combatants in the conflict [in Judges 5] against the enemies of Yahweh’s people. Although the stars are members of Yahweh’s court, the fact that the stars are noted to fight “from their courses,” םָתוֹלִסְמִּמ, indicates that they have specific positions, even portions (dare I say, “inheritances”?) in the sky that belong, on some level, to them. This also adds to the active portrait that the author is painting. Thus, the kings of Canaan have their coalition composed of kings of various cities, who fight willingly alongside each other (v. 19). Yahweh does as well, and comprises both human tribes and cosmic forces (v. 20). Since the assumption is that the stars serve as the gods of the nations (the very nations against which they are fighting), the fact that they are nevertheless cast as Yahweh’s dutiful vassals serves the Deuteronomistic agenda of the redactor quite well, which is that Yahweh’s sovereignty demands unquestioning loyalty.—Poetic Astronomy in the Ancient Near East, page 303

<idle musing>
I know—the Hebrew is backwards, but I'm too lazy to fix it...don't let that distract you from what he is saying here...The very gods that the nations think they are calling on to help them are actually bound to work against them. That's a good dose of solid theology—and a radical idea...
</idle musing>