Answer
United States v. Halper was the Supreme Court case in which Michael Dreeben, then the Solicitor General, argued against John Roberts, who was then an advocate for the petitioner. The case involved the legality of a warrantless wiretap under the Foreign Intelligence Surveillance Act (FISA). Dreeben argued that the government had met the statutory requirements for the warrantless wiretap, while Roberts argued that the government had violated the Fourth Amendment.