"They answered: 'Your servants have come from a very distant country because of the fame of the Lord your God. For we have heard reports of him: all that he did in Egypt, and all that he did to the two kings of the Amorites east of the Jordan - Sihon king of Heshbon, and Og king of Bashan, who reigned in Ashtaroth.'" NIV translation
The Gibeonites used a mixture of lies and truth to deceive the Israelites. They surely had heard about what God had done in Egypt and seen what He had done to the Amorite kings. We remember from Exodus 9 that God said that the plagues in Egypt were for that purpose.
We are reminded of the fact that this method is still used by Satan to deceive even Christians. The first step in dealing with the devil is recognizing his methods of operation.
"'And our elders and all those living in our country said to us, "Take provisions for your journey; go and meet them and say to them, 'We are your servants; make a treaty with us.'" This bread of ours was warm when we packed it at home on the day we left to come to you. But now see how dry and moldy it is. And these wineskins that we filled were new, but see how cracked they are. And our clothes and sandals are worn out by the very long journey.'" NIV translation
The story is backed up by the props that they brought with them. Many times we put more trust in our physical senses than in God and that is what the Israelites did. We must remember that it is easy to deceive the eyes. That is what magicians and the like do and it is also how the devil operates.
"The Israelites sampled their provisions but did not inquire of the Lord." NIV translation
We see that Joshua and the Israelites did not stop to pray before they did anything. They tasted the food and went with their physical senses instead of asking God about this decision.
As Christians, we must remember that our dealings with the devil must begin with direction from God.