What if the other person started it?

Being the one who didn't start it helps, and it's usually the first fact developed. But it doesn't answer everything.

Two complications.

The response has to remain proportionate. Someone who shoved you doesn't justify deadly force. Force that continues after the threat ends stops being defensive — and cases where the initial victim pursued or continued striking after the other person stopped are prosecuted.

Who started it is frequently disputed. Each person says the other did. Absent video, this becomes a credibility question resolved by whichever account the evidence better supports — which is why injuries, witnesses, and prior statements matter so much.

There's also the initial aggressor rule running the other way: a person who started it can regain the right to defend themselves by withdrawing and communicating that withdrawal.

So the useful question isn't just who threw the first punch. It's what happened after, and whether the force used stayed within what the situation justified.