In a Nov. 18, 2014 file photo, Halliburton Chairman, President and CEO David Lesar, third from right, rings the New York Stock Exchange opening bell. Halliburton and Baker Hughes have abandoned their planned $34 billion merger, the Justice Department said Sunday, May 1, 2016. The department filed suit April 6 to block the merger. (Richard Drew/AP)

When Halliburton walked away from its proposed $34 billion acquisition of oil-field services rival Baker Hughes on Monday, it became the latest corporate giant to back down in the face of Obama administration pressure recently.

Over the past year, the Justice Department has blocked a combination of media goliaths Comcast and Time Warner Cable and the Federal Trade Commission stymied plans to link up food service giants Sysco and US Foods. Treasury tripped up a tie-up between Pfizer and its Irish pharmaceutical rival Allergan. Last September, the Pentagon warned defense contractors that further mergers could raise costs to taxpayers and stifle innovation, and on April 15, President Obama issued an executive order ordering all agencies to submit within 30 days a list of actions they could take to “enhance competition.”

The Obama administration has become increasingly aggressive at enforcing antitrust rules after a relatively slow start. With Obama’s presidency nearing an end, the administration appears to be taking stock and returning to the ideas on which he campaigned.

In a 2007 speech, then-Sen. Barack Obama lambasted President George W. Bush for “what may be the weakest record of antitrust enforcement of any administration in the last half century.” But seven years into his own presidency, Obama has challenged only slightly more mergers than Bush did.

Moreover, the Obama administration has been unable to slow the pace of industry consolidation. The value of merger deals in the United States reached a new high last year of $2.3 trillion, according to the data company Dealogic. The number of transactions announced in 2012 — 12,323 — also set an record, fueled by a combination of low interest rates and sluggish economic growth.

The mergers have rolled across almost every industry, from technology to commercial dry cleaners, from airlines to beer. And more and more industries are dominated by two or three players. One merger currently under review by the Justice Department would combine Anheuser-Busch InBev and SABMiller and create a behemoth with more than 60 percent market share and a powerful distribution network.

“We did see a slight uptick in enforcement” under Obama, said Diana Moss, president of the American Antitrust Institute. But, she added, “we were hopeful at the beginning that there would be more aggressive enforcement.”

Under the last six years of Bush’s presidency, the Justice Department challenged an average 47 mergers a year. In the first six years of the Obama presidency, the Justice Department challenged 52 mergers a year. (Statistics for 2015 haven’t been issued yet.)

Obama criticized Bush for not bringing a single monopolization case. Yet his administration has brought only one monopolization case — and that case in 2011 involved a regional hospital group in Wichita Falls, Tex., accused of using its dominant market share to jack up charges to insurers for inpatient services.

“The perception in 2008 was that we would get a real sea change in antitrust enforcement,” said Daniel Crane, an antitrust expert and professor at the University of Michigan Law School. “There certainly has been an uptick of enforcement activity in the merger context; it’s been tougher to get deals through. But we have not seen a major shift in enforcement under Obama.”

The Obama administration set out to change the tone on antitrust matters in 2009, when it publicly abandoned guidance the Bush administration had given on the issue.

Yet there was relatively little chance to put a new philosophy to the test. The Great Recession made mergers difficult, and the economic slowdown made the policymakers inclined to promote activity.

In addition, in 2010, the administration altered guidelines for a technical measure of industry concentration — the Herfindahl-Hirschman Index — that made it easier for companies in concentrated markets to merge without being challenged.

Then, in 2011, the Justice Department blocked AT&T’s plan to acquire rival T-Mobile, a surprise move. “At the time, nobody thought the division was going to challenge that transaction. That set a pretty aggressive tone and has been carried on pretty consistently ever since,” said Gene Kimmelman, who served as chief counsel for the Justice Department’s antitrust division between 2009 and 2012. Kimmelman is now president of Public Knowledge, a public-interest group.

In addition to going after high-profile deals, Obama administration antitrust cops have pursued some less spectacular ones, such as stopping Thoratec, a monopoly provider of Food and Drug Administration-approved left ventricular assist devices, from acquiring HeartWare, which had the potential of entering the same market.

In March, antitrust regulators won a temporary restraining order halting Tribune Publishing’s purchase of the Orange County Register and the Press-Enterprise, after expressing concern it would constitute a Southern California news monopoly.

Tribune, which won a bankruptcy auction with a $56 million bid, argued that digital news had changed the definition of the local news market and should include competing websites.

Deputy Assistant Attorney General David Gelfand said that more than 30 proposed mergers had been abandoned during Obama’s administration as a result of challenges made or threatened by the Justice Department’s antitrust division, and the government won four other cases in court. Other companies have settled challenges by selling pieces of their businesses

The key tests for these deals typically involve a question of whether they reduce competition, potentially leading to less innovation, worse service or product quality, and raise prices for customers and consumers.

But over the past quarter-century, many economists, lawyers, policymakers and judges have taken the view that big mergers could be approved if they created cost-saving efficiencies that might benefit consumers by lowering prices.

AAI’s Moss said it hasn’t worked out well. “We have piles of evidence now across the board. All these cost-saving benefits never panned out in a lot of cases,” she said. “The whole enforcement regime has been giving excessive amounts of deference to pro-merger efficiencies without regard to less innovation, less consumer choice, lower quality products and, the biggie, higher prices from this swath of consolidation over the past 25 years.”

“So now we’re living with that legacy,” Moss said. And, she added, it has contributed to the shift in wealth away from workers and consumers because of the market influence of powerful firms. Now, she said, merger cases frequently involve tie-ups between two of the three top firms in a market because sometimes that’s about all there is left.

The Halliburton merger with Baker Hughes would have reduced competition for 23 products or services used to drill, test, seal and plug onshore and offshore oil exploration and production wells in the United States, the Justice Department said. The items included drill bits, cement and devices used to collect geophysical data.

If the merger had gone through, the new company together with rival Schlumberger would have controlled more than 90 percent of the market for eight different product and service lines, and 59 percent or more of 20 different lines.

Halliburton, which will have to pay Baker Hughes a $3.5 billion breakup fee, tried to salvage its deal by offering to peel off various business lines, but deputy assistant attorney general Gelfand said bluntly on a Monday conference call that “the deal was not fixable.”

It remains to be seen whether Anheuser-Busch InBev can pass these tests. In December, the company bought three craft breweries in five days. It has now bought eight craft breweries since 2001, a move the company might describe as providing the breweries with a greater platform but which antitrust regulators might see as gobbling up rapidly growing craft beer competitors.

Obama’s executive order asking agencies other than Justice Department and the Federal Trade Commission to come up with ideas to promote competitive markets is a recognition that more needs to be done. A report by the Council of Economic Advisers released in April said “there is evidence” of increasing concentration of ownership across a number of industries, and “decreasing business and labor dynamism.”

The Pentagon, too, has been warning about the perils of too much consolidation for months.

Last September, Frank Kendall, the Pentagon’s chief weapons buyer, said that consolidation among major defense contractors could limit competition, stifle innovation and result “in higher prices to be paid by the American taxpayer in order to support our warfighters.” He said he feared the day when the Defense Department “has at most two or three very large suppliers for all the major weapons systems that we acquire.”

Defense firms have pushed back, however, saying that it is the natural result of tightened defense spending and fewer major programs on which to bid.

Kendall’s comments were in part a reaction to Lockheed Martin’s acquisition of Sikorsky, a leading helicopter manufacturer that makes the Black Hawk, among others. He said it was the most significant change in the defense industry “since the large-scale consolidation that followed the end of the Cold War.”

But neither the Pentagon nor the Justice Department stood in the way of the deal.

An earlier version incorrectly identified the agency that stopped a combination of food service giants Sysco and US Foods. The story has been updated.

Christian Davenport contributed to this report.