A federal judge in Washington today blocked Aetna's proposed $37 billion acquisition ofHumana, punctuating an era of antitrust enforcement under theObama administration that broke up merger deals in a host ofindustries.

The U.S. Justice Department sued in July to block Aetna’s dealand also moved to block Anthem Inc.’s proposed $54 billionacquisition of Cigna Corp. Antitrust enforcers alleged the twomergers would amount to an “unprecedented consolidation in thehealth insurance industry.”

Related: Health care mergers should be blocked,says AMA

In the suit against Aetna and Humana, government lawyers allegedthe merger would hurt competition in the market for privateMedicare Advantage plans, a government-backed alternative totraditional Medicare.

“The court has some serious concerns regarding the companies’efficiencies claims,” U.S. District Judge John Bates, who presidedover the 13-day trial, wrote in his 158-page ruling. “It is very likely that a significantshare of the claimed efficiencies may be retained by the mergedfirm rather than being passed on to consumers.

Bates called the merger “presumptively unlawful” in the marketfor private Medicare Advantage plans.

He said his conclusion “is strongly supported by direct evidenceof head-to-head competition as well. The companies’ rebuttalarguments are not persuasive.”

The Justice Department had also alleged that—in 17 countiesacross Florida, Georgia and Missouri—the deal would reducecompetition for health insurance sales through the Affordable CareAct exchanges.

While the deal was still being reviewed, Aetna told federalantitrust regulators that it would reduce its presence on theexchanges in 2017 if the Justice Department sued to block theHumana deal. Aetna later delivered on that threat.

In his decision Monday, Bates found that Aetna pulled out of theexchanges in those 17 counties “specifically to evade judicialscrutiny of the merger.”

Aetna’s lawyers, led by John Majoras, a Jones Day partner, attrial defended the health insurer’s withdrawal from Affordable CareAct markets.

Majoras said Aetna made the decision not in response to thegovernment’s lawsuit but as a business decision to avoid “hundredsof millions in losses” that the insurer expected from continuedparticipation in the exchanges.

Majoras wasn’t immediately reached for comment Monday.

Hours after Trump took the oath of office Jan. 20, he signed anexecutive order that said the administration will move to repealthe Affordable Care Act.

His executive order directed federal agencies “to ensure thatthe law is being efficiently implemented, take all actionsconsistent with the law to minimize the unwarranted economic andregulatory burden of the act, and prepare to afford the states moreflexibility and control to create a more free and open healthcaremarket.”

The Anthem and Aetna cases were two of the biggest antitrustactions in Washington in years, and they highlighted an aggressiveregulatory and enforcement response to what top Obama lawyersdescribed as a “merger wave” among big companies.

U.S. District Judge Amy Berman Jackson, presiding over theAnthem trial, hasn’t yet issued a ruling. Adecision is expected soon.

The ruling Monday came as the Trump administration sets up newleadership in the Justice Department and the Federal TradeCommission, potentially presenting a new dynamic for companiesafter eight years of Democratic-led regulatory and enforcementagencies.

Some Trump advisors have argued that big doesn’t necessarilymean bad. Still, Trump hasn’t given a clear sign about how hisadministration will approach corporate mergers. Trump hasrepeatedly said he’s not in favor of AT&T Inc.’s proposedtie-up with Time Warner Inc.

"I have been on the record in the past of saying it's too bigand we have to keep competition,” Trump told Axios. “So, but otherthan that, I haven't, you know, I haven't seen any of the facts,yet. I'm sure that will be presented to me and to the people withingovernment."

In a press release from the DOJ, Deputy Assistant AttorneyGeneral Brent Snyder, who is currently heading the JusticeDepartment’s Antitrust Division, said, “Today’s decision is avictory for American consumers – especially seniors and workingfamilies and individuals. Competition spurs health insurers tooffer higher quality and more affordable health insurance toseniors who choose Medicare Advantage plans and to low-incomefamilies and individuals who purchase insurance from publicexchanges. This merger would have stifled competition and ledto higher prices and lower quality health insurance. Aetnaattempted to buy a formidable rival, Humana, instead of competingindependently to win customers. Millions of consumers havebenefited from competition between Aetna and Humana, and willcontinue to benefit because of today’s decision to block thismerger."