Saw a Monarch and a WTS go at it. The Monarch knocked the WTS from the sky and into a spider web. After the WTS got free he wanted no part of the Monarch and took off with the Monarch in hot pursuit. The Monarch came back and circled the area for about 2 minutes and left. I was under the assumption the 2 would ignore each other. Does this happen often? This is the first time I have seen it.

WTS? Is that Western Tiger Swallowtail, maybe? Butterflies do compete for food...or for mates (if males of the same species), so fighting happens. I've seen Gulf Fritillaries trying to chase both their own species and other species away quite often, since occasionally there's a lot of them around at the same time. Survival tecniques, maybe.

Yes a Western Tiger Swallowtail. Prior to this the only butterflies I have seen do something even closely resembling what happened are Skippers. This was pretty physical. Just a week ago the Monarch and 2 other WTS were all on my butterfly bush with no problems.