You don’t know someone until you’ve fought WITH them. What does “with” mean? I fought “with” you against our foe or I fought “with” you because you are my foe? It can also apply to how you and your friends relate to each other against a common opponent.
How well do you really know someone? Maybe you’ve heard of them? Maybe someone has related stories of their deeds or manners? Maybe you’ve met them, ate with them, laughed with them? There’s no pressure. There’s no pressure to win or survive. Maybe you don’t like them but you can tolerate them. Maybe you love them. You know something about them but not everything. Maybe “something” is enough under most circumstances.
You can assume you know everything about a person and there may never be a need to know more. However, people can act differently in a stressful situation. Under pressure, they may break down emotionally, fight dirty, cheat, stand their ground, surpass even their own expectations and become true heros.
I believe the saying pertains to physical confrontations as well as verbal or emotional confrontations. It doesn’t just apply to your opponents either. Your opponent might not be another human being, it might be a mountain or a house fire.
Rookies are a dangerous element to any operation. Firefighters, mountain climbers, surgeons and nurses, soldiers storming a beach. Will the rookie hold up under pressure? Can they be counted on in a crisis? Nobody, including themselves, knows how someone will react under pressure until they’ve actually been under pressure.
Just my 2 cents and you didn’t even pay that much for it.