Just as Joshua was a book about victory through obedience to God, Judges is a book about defeat because of disobedience to God. Judah and Simeon, because Simeon was given land in the center of Judah, teamed up and fought to win and control their land. For some reason, the other tribes on the west side of the Jordan River did not attempt to conquer their land. Maybe they were tired of years of war or maybe it was because they were not trained by their parents to serve the Lord. 10 After that generation died, another generation grew up who did not acknowledge the Lord or remember the mighty things he had done for Israel.11 The Israelites did evil in the Lord’s sight and served the images of Baal.12 They abandoned the Lord, the God of their ancestors, who had brought them out of Egypt. They went after other gods, worshiping the gods of the people around them. And they angered the Lord.13 They abandoned the Lord to serve Baal and the images of Ashtoreth. Judges 2:10-13 (NLT)
Judges cannot be studied as a sequential history. In chapter 11, we read the Israelites had been living in Gad for 300 years which would place the time around 1100 B.C. Chapters 1-16 deal with the nation as a whole while chapters 17-21 deal with individual tribes.
A good outline of Judges might be:
1A. The explanation for failure. Ch. 1-2
2A. The work of the judges. Ch. 3-16
3A. The total failure of the people. Ch. 17-21
A great theme for Judges is the last verse (and key) verse of the book. 25 In those days Israel had no king; all the people did whatever seemed right in their own eyes. Judges 21:25 (NLT).