How is the theme of national unity highlighted in the book of Joshua, for instance, in stories like the construction of an altar by the Transjordan tribes?
From the beginning of the book of Joshua, the Lord asserts that all Israel be consecrated before the Lord, and all Israel be prepared for war, and all Israel be united in worshiping the Lord. That’s why, for example, in chapter 22 we see that the tribes to the east of the Jordan — the Reubenites and the Gadites and the half-tribe of Manasseh — built for themselves an altar as a witness before the Lord. The rest of the tribes, on the west side of the Jordan, thought that these tribes had rebelled against the Lord by building an altar other than the one in the tabernacle of Moses. What happened is that Joshua addressed this issue by sending Phinehas, the son of Eleazar the priest, and sent with him ten chiefs of Israel to evaluate the situation. The result was a reassurance of the unity of all Israel in their worship of the Lord. Consequently, the unity of Israel — all Israel together — was very important regarding conquering the land, distributing the land, and worshiping the Lord. It was a very important issue before the Lord.