“She started it” stopped working at age seven. Find a newer move, like “I made it worse.”

Who began something is almost never the useful question. It is a very good question for avoiding the part you added, because the start is the one piece you can prove was not yours. Meanwhile the escalation, the reply that raised the temperature, the thing you said next, were all choices made with full information.

In the next argument, skip the origin story completely and name your own contribution to how bad it got. One sentence is plenty. It also tends to disarm the other person faster than any evidence about who moved first.