The Justice Department filed 138 antitrust cases in federal courts from 1999 to 2008 and lost just four of them, according to the latest breakdown from the agency.

One reason that the Justice Department has such a good track record is because it rarely challenges a deal unless it’s very confident it can win, said Joseph Bauer, a University of Notre Dame law professor and antitrust expert.

Knowing AT&T would probably go to court, the Justice Department might have wanted to signal that it intends to get tougher on corporate marriages between rivals in markets with few other competitors.

A union between AT&T and T-Mobile USA would leave Verizon and Sprint as the only other major cell-phone carriers in the U.S. T-Mobile, a subsidiary of German telecom company Deutsche Telekom AG, is currently the No. 4 wireless carrier, while AT&T is second. Combined, AT&T would be the largest.

In a sign of its confidence, the Justice Department decided to strike down the deal even though it could have taken about three more months to study the pros and cons. The timing stunned AT&T, which said it didn’t get any advance warning.

“It was an aggressive and impressive move by the DOJ to take the battle right at AT&T,” said Daniel Wall, a San Francisco attorney who represented Oracle in its 2004 fight to win the right to buy PeopleSoft. “It sent a statement that the DOJ intends to fight this one all the way to the finish line.”

Wall said AT&T might have a tougher time proving its case than Oracle did against the Justice Department. In the PeopleSoft deal, Wall said, antitrust enforcers seemed to be manipulating the definition of the business software market. “This time, it looks to me that they have a pretty solid market definition,” Wall said. “They don’t appear to be playing games.”

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