After they had done their duty and helped the rest of the tribes secure their land, the tribes of Reuben, Gad, and the half tribe of Mannassah got to go home. Moses had let them claim the lands they had already conquered east of the Jordan River. When they got home, they decided to build a large replica of the official altar of the Tabernacle as a sign to future generations that they were part of the people of God.
The rest of Israel heard this and concluded they were going to start having unauthorized sacrifices. This was not the case. It was purely symbolic, and the war got called off.
It sometimes seems plausible that our brothers in other denominations are engaging in some gross abomination. Upon closer observation, we might discover that we are wrong. But first we must do the examination. Mere jumping to conclusions leads to war.