According to a surprisingly small number of atheists, iron chariots are the Lord's kryptonite. Citing Judges 1:19, they claim God was too weak to overthrow the people living in the valley, which means if God really exists, the Bible exaggerates His power.
As usual, context is key. First, it is significant that the KJV is the translation most often cited here. Although other versions do present a similar problem, translations like the HCSB and the NIV make it far clearer: God was not the one too weak to fight the chariots.
The mistake unbelievers make here is assuming that if God is with someone, they must necessarily win every battle, defeat every foe, and flawlessly succeed in every endeavour. In reality, God has a purpose for every apparent "failure". Joseph was taken into slavery while the Lord was with him, was he not? He is just one example wherein the Lord's presence did not achieve immediate victory, but achieved a greater purpose. Put simply, if God wanted the Jews to take Israel all at once, He could have, and He would have. The problem? He explicitly stated He had no intention of doing so.
In Exodus 23:29-30, God tells the Israelites that He would not allow them to completely drive out the inhabitants of the land in a year. Rather, little by little, He would drive them out. His stated reason for this is that they were not yet numerous enough to make full use of it. With so few people maintaining the land, it would become desolate, and a haven for wild animals. So, God would not drive out all the inhabitants of the land until they were numerous enough to completely occupy it, which we have no reason to believe they would have done between the time God made this declaration and the time Judah failed to take over the valley.
It even makes sense that God would leave the strongest inhabitants in the land rather than the weakest. For one thing, if the intention is to stop wild animals multiplying, which at this time would have included some very fierce creatures from lions to bears to maybe even dinosaurs, it makes sense to leave the inhabitants that stood a fighting chance against such creatures.
Second, word spread surprisingly quickly back then. We know for a fact that the Israelites were notorious for being followed by God. Cities were fortifying themselves against them, and when they came to Jericho, Rahab already knew what was going to happen. Because word spread so fast, animals would have been the least of the survivors' worries. They would be fighting a war on three fronts. They would be defending against Judah and his armies, they would have been striving to keep the predator population down, and they would have been fighting off nations who heard of their defeat and wanted to plunder what they had left. No iron chariots = mission failed.
A single Bible verse, especially when presented by a hostile source who likely has not even read the actual chapter, is never enough for full context. The Bible must be studied in depth to get a full understanding of it.