https://en.m.wikipedia.org/wiki/Ironfest_(Lithgow)
That gives a basic rundown of the event itself.
Every year there are a bunch of different reenactment/HEMA groups ranging from Romans to various stages of the medieval era to WWII, and over the weekend they have fights to show off their groups and try to get more people into it.
They generally only fight themselves (so the viking group doesn't fight the Roman's for example) due to safety and possible differences in equipment standards, but during the WWII fight each year the others always charge onto the field to join in. Last year they did it at the end and got massacred by the MGs on the tanks - watching 30 guys in full armor go down in the most over the top ways is hilarious live btw - but this year they split into two, and took the germans and yanks/Aussies from behind halfway through their fight, then climbed into some the trucks and half tracks and did a victory lap of the field.
If l can upload the pics I have somewhere trustworthy I'll share em