The borders of Jordan, that are in - Canaan - This verse can never mean that they built the altar on the west side of Jordan, for this was not in their territories; nor could it be a place for the purpose of public worship to their own people, if built on the opposite side of Jordan; besides, the next verse says it was built over against the land of Canaan. It appears that when they came to the river they formed the purpose of building the altar; and when they had crossed it they executed their purpose.
A great altar to see to - A vast mass of earth, stones, etc., elevated to a great height, to serve as a memorial of the transactions that had already taken place. Probably it was intended also to serve as a kind of watchtower, being of a stupendous height, altare infinitae magnitudinis, an altar of an immense size, as the Vulgate terms it.

And when the children of Israel heard of it,.... Of the building the altar in the above place, namely, the nine tribes and a half settled in the land of Canaan:

the whole congregation of the children of Israel gathered themselves together at Shiloh; where the tabernacle and altar of the Lord were, whose cause they undertook to avenge, being injured as they imagined by the altar the other tribes had built, and where they could consult the Lord by Urim and Thummim, if needful; hither they repaired from the several places around, where their tribes were settled; not the whole body of the people, but their heads and representatives:

to go up to war against them; to consult about it, and to prepare for it, which they were obliged to do by the law of God, as in the case of a city, so of a tribe drawn aside to idolatry; and which they imagined was the case of these tribes, or at least what they had done had a tendency to apostasy from the true worship of God, which they were zealous to defend at the hazard of their lives, and though it should issue in an extirpation of one or more of their tribes; see Deu 13:12.

CONTENTION THEREUPON. (Jos. 22:11-29)

and the children of Israel heard say--Fame speedily spread intelligence of what the trans-jordanic tribes had done. The act being suspected of some idolatrous design, the tribes rose in a mass, and repairing to the tabernacle at Shiloh, resolved to declare war against the two tribes and a half as apostates from God. On calmer and more mature consideration, however, they determined, in the first instance, to send a deputation consisting of the son of the high priest, and ten eminent persons from each tribe, to make inquiry into this rumored rebellion against God (Deu 13:13-15). The quality of the deputies evinced the deep solicitude that was felt on the occasion to maintain the purity of the divine worship throughout Israel. In the presumptive belief that the two tribes and a half had really built an altar, the deputies expressed astonishment at their so soon falling into such a heinous crime as that of violating the unity of divine worship (Exo 20:24; Lev 17:8-9; Deu 12:5-13). They reminded their eastern brethren of the disastrous consequences that were entailed on the nation at large by the apostasy at Peor and by the sin of Achan, and finally exhorted them, if they felt the want of the tabernacle and altar and repented of their rash choice in preferring worldly advantages to religious privileges, to remove to the western side of the Jordan, where all the tribes would form a united and obedient community of worshippers.