"The LORD was with the men of Judah. They took possession of the hill country, but they were unable to drive the people from the plains, because they had iron chariots." — Judges 1:19
So, Judah had God on his side, which is pretty kick-ass, so he and his buddies do a lot of ass kicking. With the omnipotent, omniscient creator on their side, they were just whoopin' Canaanite and Perizzite tail. They killed 10,000 in Bezek alone, burnt Jerusalem, and cut off Adonibezek's thumbs and big-toes. NOBODY was left standing in their path...
...until they got to the valley. Because, after all, those people had iron chariots for, uh...God's sake.
Yeah. The most powerful entity in the universe can't fuck with iron chariots. Even though he made iron. And chariots. Flooding the planet? No problem. Pillars of fire and gaping holes in the earth? Easy. I mean, you'd think he could at least kill the horses, leaving the people trying to kick-push the chariots Flintstone or skateboard style. But no. God turns completely flaccid at the sight of iron chariots. Kinda like me and Star Jones.
I still think I could whoop Star's ass, though, and I can't even open those windows in the sky to let it rain.
But at least he gave them fair warning:
"but the forested hill country as well. Clear it, and its farthest limits will be yours; though the Canaanites have iron chariots and though they are strong, you can drive them out." — Joshua 17:18
Or that could just be one of those pesky contradictions. Or God lying. Or changing His omniscient mind. Or just the ancient ramblings of a superstitious Bronze-Age goat herder.