The first major section in the book (1:1-3:6) explains very clearly why the period of the judges was a dark chapter in Israel's history. God revealed the reasons for Israel's apostasy and consequent national problems in terms too clear to miss.
The years immediately following Joshua's death saw a transition from success to failure. The events of this period set the scene for the amphictyony (rule by judges) and provide a background for the main part of the book (3:7-16:31).
"The Book of Judges may be viewed as having a two-part introduction (1:1-2:5 and 2:6-3:6) and a two-part epilogue (17:1-18:31 and 19:1-21:25). Parallel ideas and motifs link the first introduction (1:1-2:5) with the second epilogue (19:1-21:25), and in like manner the second introduction (2:6-3:6) with the first epilogue (17:1-18:31)."19