The answer is: Built an altar

Josh 22:10-12 - They thought the altar was built to worship other gods and so were ready to go to war until they realised it had been built as a witness to the true God.

In the Book of Joshua, the conflict between the tribes east of the Jordan and their western counterparts gives us a fascinating peek into early Israelite culture and their understanding of communal worship. The altar built by the eastern tribes near the Jordan River wasn’t just about fabricating stones; it represented a significant misunderstanding among the tribes. What the west initially perceived as a possible rebellion against Yahweh turned out to be a shared agreement to remember their faith and connection to God.

This incident reflects a deep-seated need for unity in worship and recognition of the same God. Interestingly, altars were a major part of ancient Near Eastern culture, serving as places for sacrifices and offerings. In many ways, this solitary altar symbolized a boundary, not just between the tribes geographically but also in worship practice. They feared that separation could lead to worshiping different gods. As it turns out, their fears were larger than the reality; it was instead a sign of faith in the same deity—a diplomatic gesture rather than an act of defiance.

It’s intriguing to note how easily misunderstandings can escalate into conflict. This narrative carries an essential lesson about communication and the importance of clarity in expressing faith. It's a poignant reminder about how jumping to conclusions can prematurely ignite crises, teaching us the value of seeking to understand the intentions and reasons before malice brews.