In Joshua 9 we see what happens when we rely on our own ability without consulting God. Joshua, though aware of the possibility of deceit, did not inquire of God (though he had been instructed by Moses to do exactly that in Numbers 27:21) and ended up being tricked. He relied on his own wisdom instead of seeking the wisdom of God, and paid the price for it. He did, however, keep his oath, even though it had been made in haste and by trickery. God takes promises seriously and we would be wise to to think before uttering any rash promises!
However, this is also a story of how when sin abounded, grace abounded all the more. Joshua assigned the Gibeonites work where they would be near the temple and exposed to the truth about God. Eventually, many of the Gibeonites became Israelite believers themselves (1 Chronicles 12:4, Nehemiah 3:7, Nehemiah 7:25). It's amazing how God works despite our sin.
What I noticed in the rest of our passage was Joshua's strengths as a leader - his determination to obey everything God and Moses had instructed him to do, and his willingness to get his hands dirty himself, and not just lead from afar. But also, in every victory credit is given to God.
It's also important to note that though we read through this passage in one sitting, the process of ridding the land of the Canaanites took a long 7 years of hard fought battles. We need endurance, we need patience, we need determination, we need energy - and we need to acknowledge that it all comes from God and not our own strength.
Tomorrow's Bible In a Year Passage: Joshua 12-15