“Now Jephthah.” Judges 11:1 is the direct continuation of Judges 10. The immediate context of Judges 11:1 is Judges 10:17, so for the best understanding, start reading the Jephthah record with Judges 10:17.
“was a valiant warrior.” Judges 11:1-3 are almost parenthetical, and introduce Jephthah. He was a valiant warrior, something that had to have been demonstrated over time by his leadership and success in raids and skirmishes, nevertheless, he is a social outcast, which is explained in these verses. Judges 11:4 then picks up where Judges 10:17 left off, with the war between Israel and Ammon.