Echoing the general sentiment here. If there's no choice, don't give them a choice to start.

An example I used before is when the campaign is about two groups at war, and the intro session is the last negotiation before things break down into war.

In this case, the players should be told "No matter what you do, war is gonna break out. Doesn't matter how good your checks are, how convincing your arguments are, or anything else-war is gonna happen. Use this time to gather info, make allies, set yourselves up with a good reputation, but out of character, your stated goal of peace is not happening."