Secret War.

He learned that each side will form a team that they will send into battle. The teams will be equal in size and will fight until one of them is eliminated or until one side decides to interfere.

The side that decides to interfere will send another team to help their losing team. This will cause the other side to send in another team to assist their winning team. This will continue until one side doesn't have teams to send anymore. 

One important rule is that anyone on the battlefield can leave at any time. They can leave to rest, but they can only return with another set of teams, or they can choose not to return at all.

In the meantime, their teammates on the battlefield will be down a man and might become outnumbered. If they become outnumbered, they will face defeat. This will prompt their side to reinforce them early. But then the other side will also be able to send in more men to press their advantage.