As one reads through the book of Joshua, two main themes pop out: 1) God always keeps His promises and 2) Joshua and the Israelites as a whole were very careful to keep the Law of Moses. Almost everything in the book revolves around one of these two themes and the themes intertwine.
Joshua's fidelity to the Law can best be seen in the minutiae of the law. No one is surprised that he ordered the march across the Jordan as commanded nor that he gathered the people to Shechem to read the law and erect the memorial as Moses taught (Deut. 27:1-8). What is surprising, perhaps, is that Joshua remembered the "minor" details even in the heat of battle or the rush of victory.
Josh. 10:24, 26-27 "And it came to pass, when they brought forth those kings unto Joshua. . . . Joshua smote them, and put them to death, and hanged them on five trees: and they were hanging upon the trees until the evening. And it came to pass at the time of the going down of the sun, that Joshua commanded, and they took them down off the trees, and cast them into the cave wherein they had hidden themselves, and laid great stones on the mouth of the cave, unto this very day."
He hung them in trees on display, and then tossed their corpses in a cave and sealed it. So what? Everything he did there was right out of the Law, that's what:
"And if a man have committed a sin worthy of death, and he be put to death, and thou hang him on a tree; his body shall not remain all night upon the tree, but thou shalt surely bury him the same day; for he that is hanged is accursed of God; that thou defile not thy land which Jehovah thy God giveth thee for an inheritance." (Deut. 21:22-23)
Joshua had them cut down before sunset (same day) cast in a cave and sealed (buried) so the land would not be cursed. How many God-fearing leaders in Israel's history would even have been aware of that command, much less remembered it after the longest single day battle there ever could be?
This sums up Joshua's every action as recorded in this book. He did make mistakes -- the Gibeonites come to mind -- but he was diligent in following the Law. Even the people were obsessed with following God. They were angry with Joshua and the leaders because they couldn't follow God's commands due to the vow made to Gibeonites (9:18) and almost began a civil war when they thought the tribes east of the Jordan were building their own altar in contravention of the law (chapter 22).
While Joshua and the people were following God's commands, God was busy keeping His promises. The people got the land promised to Abraham and became a great nation. God fought for them (Deut. 3:22). God magnified Joshua (Josh. 3:7; 4:14). God gave them cities already built, wells already dug, and vineyards already planted (Deut. 6:10-12; Josh. 11:13; 24:13). This is all summed up by Joshua when he says, "you know in your hearts and souls, all of you, that not one word has failed of all the good things that the LORD your God promised concerning you. All have come to pass for you; not one of them has failed." (Josh 24:13)
Of course, the intertwining of these themes is in the provisional promises of God. While the Abrahamic promises were not conditional, those to fight with Israel and magnify Joshua were. If, God says, you follow me, then I will do these things. In many ways the period under Joshua was a Golden Age because the people did follow God, and therefore He rained blessings on them. That object lesson is the theme of this whole book.
Lucas Ward