This amount of chariots was an impressive force, especially difficult for infantry. These chariots were already mentioned as a problem earlier in the book, in Judges 1:19 and Judges 1:34–35; they were the reason that Judah and Dan could not dislodge the Canaanites from where they were in the southwest. Israel did not possess this technology. And the Jezreel Valley would’ve provided an ideal, level terrain for chariotry. Israel, when not leaning upon the Lord, is no match for Sisera.
But Chisholm rightly asks, “Since when have chariots been able to thwart God’s purposes and power? The Lord destroyed the Egyptian chariots in the Red Sea (Exodus 14:23–28; Exodus 15:4). He promised to give the Canaanite chariots into Israel’s hands and instructed Joshua to burn them (Joshua 11:4–6), orders which Joshua carefully obeyed (Joshua 11:9). Later Joshua assured the men of Joseph that the Canaanite iron chariots would not prevent them from conquering the plains (Joshua 17:16–18)."1
3 Then the people of Israel cried out to the LORD for help, for he had 900 chariots of iron and he oppressed the people of Israel cruelly for twenty years.