Today's reading is Joshua 22-24 (read it in the KJV or NIV)
Today's passage covers the eastern tribes' return home and their construction of a new altar; the altercation (no pun intended) over it; Joshua's farewell to the leaders; the renewal of the covenant at Shechem; and Joshua's death and burial.
The two and a half eastern tribes -- Reuben, Gad, and half of Manasseh -- have been away from home for a long time. They valiantly fought alongside their fellow Israelites, dutifully conquering the promised land of Canaan, and never once did they complain, at least not on record. By Josh. 22, it's clear that the little pieces of land left to be conquered will be left up to the individual tribes, and it's time for the eastern tribes to return home. Joshua blesses them, thanks them for a job well done, and sends them on their way. (Josh. 22:1-9)
The first thing they do when they get home is build an altar. (Josh. 22:10)
This isn't any ordinary altar, either. It's a "great" (KJV) and "imposing" (NIV) altar. (Josh. 22:10) Moreover, it's a replica of the main altar that goes along with the ark of the covenant, the one we read so much about in Exodus. (Josh. 22: 28)
This is, to say the very least, suspicious. After all, they have just been conquering pagan lands, and it's possible that the Reubenites, Gadites, and Manassehites have picked up the ways of the conquered peoples. So the rest of the Israelites send a delegation to see what the eastern tribes were thinking when they set up this huge altar. In particular, they send ten Israelite princes and Phineas, son of the high priest. We may remember Phineas from Num. 25, when he dealt with an Israelite man and his Midianite concubine by stabbing them both through with a spear. Clearly, the Israelites are ready for trouble. (Josh. 22:13-14)
The delegation arrives and, as we might expect, challenges the eastern tribes. If they've learned nothing else over the years, they've learned that God punishes the entire congregation for the faults of the few. "Remember Peor," they say (Josh. 22:17), in reference to Num. 25, when some men followed their Moabite women in worshipping a foreign god, and God sent a plague that killed 24,000 people. "Remember Achan," they say (Josh. 22:20), in reference to Josh. 7, when Achan stole some of the sacred items from Jericho, and God caused the Israelites to be routed at Ai. Knowing that God is jealous and quick to anger, they correctly reason that if the eastern tribes have made God angry, their sin will quickly translate into punishment for the rest of the congregation. They even tell the eastern tribes that, if the temptation for idol-worship is too great, they can make room west of the Jordan for them, and all Israelite can live together as one big, happy family. (Josh. 22:15-20)
The eastern tribes are quick to leap to the defence. "If we transgressed, God will know and you will be right to punish us," they say. (Josh. 22:21-23, my paraphrase) No, they did not build the altar as a place to offer sacrifices and offerings, but instead as a precaution against the rest of the Israelites. They reason that one day in the future, the main group of Israelites will claim that God only gave Israel the lands west of the Jordan, and cut off the eastern tribes. This altar will not be used for sacrifices, but will be a witness between the western and eastern tribes, so that all their descendants will realize they are part of the same people and worship the same God. (Josh. 22:24-29)
This answer is good enough for Phineas and the princes, who return and tell the rest of the Israelites why the eastern tribe set up an altar. The congregation like the answer too, and there is no more talk of going to war against their eastern brethren. (Josh. 22:32-33)
This is, to say the least, a masterful piece of reasoning on the part of the eastern tribes. While the thing they built might look like an altar and act like an altar, it is not, in fact, an altar. At least, it's not an altar for sacrifices, which is the general purpose of an altar. Instead, it is more like a primitive national flag, a way of saying that the eastern tribes belonged to the same family as the western ones.
More fundamentally, it is the precursor to a culture based more on law than on actual location. In the diaspora, after Canaan was conquered by the Babylonians and Persians, Jews developed a religion based on Torah scrolls as opposed to animal sacrifices at a central temple. Judiasm, and later Christianity, could spread throughout the world, never tied to a single locale or a single temple. While the main movement towards book-based (as opposed to sacrifice-based) worship occurred much later, this chapter places the seeds for that later development. Even before the advent of a Temple, the Israelites were already starting to set up contingencies for people who were too far away to travel to it.