It's a long story. I wasn't there for most of it, and a lot of it is stuff I had to work out after.

Basically, because of several very destructive incidents that the Avengers played a role in, even if they didn't exactly cause them, the UN decided they should set a up a panel to oversee the team. Mr. Stark was for it, Captain Rogers was against it, apparently because he felt it would limit there ability to help people? The other Avengers sort of took sides from there. It probably wouldn't have been a thing I ended up involved in, but then a guy whoo looked like Bucky here caused an explosion that killed the King of Wakanda during the signing of the Accords that created the oversight panel.

Cap went after Bucky, figuring he'd framed, which he was, which kind of violated the Accords. It turned into this whole huge mess, one half of the Avengers fighting the other half, and I ended up involved because Mr. Stark figured he needed some backup, and kinda-sorta blackmailed me into fighting by threatening to tell my aunt what I'd been doing.