“And Judah overthrew Azzah with its territory and Ashqelon and its territory, even Eqron and its territory. And Yahweh was with Judah and he took possession of the mountainous region. He could not take possession of the dwellings of the plains for they had chariots of iron.”
The note about chariots of iron is actually tremendously helpful when it comes to confirming the dating of these historical events recorded in Joshua and Judges. As scholars and archaeologists examine the development of civilizations, they typically date the beginning of the Iron Age around 1200 or 1300 BC. Obviously there is not a hard and fixed date as cultural changes in terms of the usage of metals for tools and warfare, these changes are gradual, but in the 13th century B.C. studies have shown the increased use of iron instead of bronze.
At the same time, most conservative Biblical scholars place the conquest of Canaan between 1375 or 1350 BC. This would mean that these Canaanites of the plains were a bit ahead of their time in building iron chariots, but it would also explain why Judah and Simeon had such difficulty when seeking to conquer these peoples. This was a radical development and weapons of bronze were simply no match for weapons of iron and such was the plight for the armies of Judah as they engaged the peoples of the plains. But for those who doubt the historicity of the Scriptures, this is just one of numerous reminders that the Biblical account is quite consistent with the discoveries in the world around the ancient near east.
This, though, marks the first of the inhabitants of the land that the armies of Israel were unable to drive out — the first of many. Those people in the land would cause great grief for the people in the long run, but we get ahead of ourselves.