In connection with the apparent failure to overcome superior military technology, the New American Commentary (Judges, Ruth; p.100) says
However, the significance of the author's reference to the Canaanites' iron chariots lies in the theological implications of Judah's inability to overcome superior technology. In the light of Deut 7:1 - 3 and after the miraculous conquest of Jericho (Joshua 6), no one, no matter how technologically superior to the Israelites, should have been able to withstand Judah's attack. This verse must be read in the light of Josh 17:16 - 18, according to which Joshua had encouraged Ephraim and Manasseh by specifically declaring that the Canaanites' superior strength and their possession of iron chariots would be no hindrance to the Josephite tribes' conquest of the river valleys and plains. In our text (v.18a) the narrator explicitly attributes Judah's successes in the hill country not to equivalent military power but to the presence of Yahweh. Then why could they not take the lowland? Why is Yahweh's presence canceled by superior military thechnology? The narrator does not say, but presumably the Judahites experienced a failure of nerve at this point, or they were satisfied with their past achievements.
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I can think of other examples that have a similar MO: The disciples inability to cast out a demon (Mt 17:14ff), Peter's inability to continue walking on water (Mt 14:28 - 31). Compare also Ex 15:25b - 26; 16:4, 5; Deut 8:2 - 5; 13:1 - 3; Jdg 3:1 - 6.