What does Judges 1:3 mean?

After Joshua's death (Joshua 24:29–31), Israel's new mission was clear. As they had been told by Moses (Deuteronomy 7:1–5; 9:4), they were to drive all the wicked Canaanite peoples out. Each tribe of Israel was to take complete possession of the territory apportioned to them. The people have asked the Lord which tribe should attack first. God responded that Judah (Genesis 49:8–9) should lead, promising to give Judah's territory into its hands (Judges 1:1–2).

This verse depicts the conversation between the tribes of Judah and Simeon as if the patriarchs themselves were speaking. The leaders of Judah's tribe invite the leaders of Simeon's tribe into an alliance. If Simeon will come and fight with Judah for Judah's territory, then Judah will fight with Simeon's people to clean the Canaanites out of their land. Simeon's people agreed.

This arrangement makes excellent sense for two reasons. First, Judah's tribe is large and has been given the largest single territory of all the tribes, on the southern end of the Promised Land. Simeon's tribe is the smallest of the others by far. It will need help. Second, Simeon's territory exists within the borders of Judah's territory. By default, Judah will end up serving as Simeon's protector. It follows that their fighting men should join forces.