The U.S. Department of Justice’s first witness in its challenge to AT&T’s proposed merger with Time Warner testified Thursday, warning that if the two companies merge, they would have significantly more leverage in negotiations.

The  antitrust trial, expected by many to be the most influential in decades, began in earnest Thursday with the testimony of Cox Communications Inc.’s Suzanne Fenwick, a content acquisition vice president, as well as opening arguments from lawyers for AT&T and the DOJ.