Sefer Shoftim opens with 2 separate introductions. The first chronicles the gap between the Jewish leadership of Yehoshua until the beginning of the Shoftim. Indeed the first Perek (up to Perek 2 Possuk 5) discusses many different battles that began during the lifetime of Yehoshua. Some of these battles were fought by the national armies and others by Shevet Militias. One thing unites the battles though --- that they were all battles fought with an eye toward the betterment of the national unit that is Am Yisrael. Notwithstanding, a united nation without the word of Hashem is only half the job and thus the warning of the need to follow Hashem and his word at the beginning of Perek 2. This introduction to the Sefer serves the purpose of bridging the gap and telling us why it was that the nation never completed the conquest of the land of Israel.

 The second introduction appears to set up the cycle that gides most of Sefer Shoftim -- it seeks to explain a) why the Jews find themselves in the hands of their enemies b) Why they will be saved time and again c) Why they don't annihilate their oppressors when they ARE saved and d) why Yehoshua never captured everyone to begin with. 

In the end, the Eliyahu Rabba (30) notes that Hashem allows the nations of the world to persecute the Jews "a little" when we misbehave but the oppressers take their jobs too seriously and as a result, are subjected to Hashem's wrath.