Make War is a gameplay mechanic in Divinity: Original Sin 2. It is a new feature that was added during early access and allows for some unique roleplaying approaches.
The new Make War mechanic much as the name suggests, allows you to turn your former friends into enemies. When you select this option, you will become enemies with your ex-party members and will enter turn-based combat with each other whenever you approach. Whoever flees or dies loses. If you don't want to fight your ex-party members, but want to be sneaky, the option allows you to prevent them from seeing your quest journal.