Why are the Socs and the Greasers going to fight in the vacant lot?

From the text:

"Don't worry," Dally went on, "it ain't loaded. I ain't aimin' to get picked up for murder. But it sure does help a bluff. Tim Shepard's gang and our outfit are havin' it out with the Socs tomorrow night at the vacant lot. We got hold of the president of one of their social clubs and had a war council. Yeah"--- Dally sighed, and I knew he was remembering New York--- "just like the good old days. If they win, things go on as usual. If we do, they stay outa our territory but good. Two-Bit got jumped a few days ago. Darry and me came along in time, but he wasn't havin' too much trouble. Two-Bit's a good fighter. Hey, I didn't tell you we got us a spy."