After the death of Joshua, the people of Israel enquired of the Lord, “Who shall go up first for us against the Canaanites, to fight against them?” The Lord said, “Judah shall go up; behold, I have given the land into his hand.” And Judah said to Simeon his brother, “Come up with me into the territory allotted to me, that we may fight against the Canaanites. And I likewise will go with you into the territory allotted to you.” So Simeon went with him. Then Judah went up and the Lord gave the Canaanites and the Perizzites into their hand, and they defeated 10,000 of them at Bezek.
At start of the book of Judges we meet the nation of Israel facing a leadership crisis. Joshua their greater leader after Moses has died. This is a huge problem of course because even though the they have managed to break the back of the Canaanite resistance they not done enough to fully possess the land. The enemy is weakened but they now need to go in and do the dirty work of fully possessing it.