Befriend The Enemy

Why can't people do this more often in real life?

When someone decides to stop being enemies with their foes and start being friends. Of course, it may be a little hard to crack them, so the friendly one has to bring up a point that would make the enemy want to join them. Two of these reasons can be the Common Threat can harm them both and they need to team up to take it down or it'd be better to travel in a larger group working together.