But this week, the Obama administration took on both deals - and it has claimed at least one trophy so far.

Pfizer walked away from its proposed merger with Allergan on Wednesday, complaining that new tax rules from the Treasury Department sharply curtailed the benefits of that deal.

And the Justice Department sued Halliburton and Baker Hughes on Wednesday, arguing that putting the two together would cut competition in the oil field services industry to unacceptable levels. The two companies plan to fight the suit.

In both cases, the government showed how it has been increasingly willing to challenge giant takeovers, reflecting a belief that the corporate world goes too far in its pursuit of megamergers.

"The concern we have is the leaders of some corporations who are looking to take the best of America without making a contribution to the success of our country," Josh Earnest, the White House press secretary, said Wednesday when asked about Pfizer and Allergan. "That's wrong."

The government's challenges raise questions about the strength of what had appeared to be a surging market for mergers, particularly after a banner year in 2015. Transactions valued at about $US4.7 trillion were announced last year, according to Thomson Reuters data, smashing records.

Abandoning large-scale mergers

Recent moves by the administration, though, are already tempering plans for big, risky transactions, lawyers and bankers say. Monsanto, which last year had pursued a failed takeover of a rival that analysts cautioned could draw antitrust scrutiny, told investors Wednesday that it was abandoning "large-scale" mergers as a strategy.

"A board that's considering a transaction has to reassess what are the outer limits of what the government is prepared to do," said Richard Brand, a mergers and acquisitions partner at Cadwalader, Wickersham & Taft. "If it's a politically unpopular transaction and the government has the tools to block it, they might block it just because they can."

The recent spate of activity from regulators is undoubtedly in part because of the higher level of deal-making. But supporters of the US administration's efforts also point to what they say are further attempts to consolidate industries that are already concentrated in the hands of a few companies.

"It's not that the government has suddenly decided to become more aggressive," said Diana Moss, the president of the American Antitrust Institute, a nonprofit that favours tougher antitrust enforcement. "We're coming out of a phase where a lot of mergers were let through."

Administration officials denied that they were now increasing the pressure.

Risky deals

In a conference call about the lawsuit against Halliburton and Baker Hughes, the head of the Justice Department's Antitrust Division, William Baer, said that he was not trying to be more aggressive in trying to block mergers. Instead, he said, companies have been striking more deal, and betting that they will not have to make large changes to get those transactions approved.

"We are not changing how we look at or analyse mergers," he said. "There are some deals that are so antitrust-risky that they never ought to make it out of the executive suite or the corporate boardroom."

Obama-era regulators have blocked a number of blockbuster mergers, including AT&T's $US39 billion effort to buy T-Mobile USA and Comcast's $US45 billion bid for Time Warner Cable.

The Federal Trade Commission blocked an attempt by Sysco and US Foods to merge, which would have united two of the nation's biggest food service providers.

Not all of the Obama administration's antitrust efforts have succeeded. Despite initially suing to block the merger of American Airlines and US Airways, the Justice Department eventually withdrew its lawsuit and allowed the two to combine, creating the country's biggest airline.

And some commentators have said that the FTC's lawsuit seeking to block the merger of Staples and Office Depot has hit roadblocks, with the regulator facing questions from the judge presiding over the case.

Still, when it comes to deals known as inversions, which allow a US company to move its headquarters overseas to cut its tax bill, the Obama administration has tried to draw a line in the sand.

Line in the sand

In its first two attempts, starting in 2014, the Treasury Department was largely unsuccessful in thwarting the wave of inversions, or in persuading Congress to act. Companies like Johnson Controls and the research firm IHS continued to sign deals that let them to leave the United States.

But Treasury's latest rules, announced Monday, sent shock waves across corporate America.

One big piece was aimed at what the administration called "serial inverters," or corporations that turned into bigger ones by conducting multiple deals for tax efficiencies.

It appeared to be aimed squarely at the Pfizer-Allergan deal, which, while technically not an inversion, would have had about the same effect.

Allergan's operations are largely in New Jersey, but its tax domicile is Ireland, thanks to a series of deals, starting in 2013.

Before the Allergan deal, Pfizer, a 167-year-old company founded in New York City, had been vocal about abandoning its US citizenship for lower taxes, having tried to acquire AstraZeneca of Britain for $US119 billion in 2014.

Exhibit A for corporate greed

So when it announced the Allergan deal in November, it reignited an uproar. Presidential candidates from both parties railed against the merger, which became Exhibit A for corporate greed and tax avoidance, in an election season full of talk about breaking up big companies.

Pfizer and Allergan had expected the Obama administration to take some sort of action to trim the financial benefits of their deal. But the latest batch of rules stunned the two companies.

In an interview on CNBC on Wednesday morning, Brent Saunders, the chief executive of Allergan, said that he felt blindsided.

"We viewed this as a remote possibility," he said. "Unfortunately, we were wrong."

Saunders also accused the administration of playing unfairly in trying to stop the deal. "For the rules to be changed after the game has been played is a bit un-American," he said.

Earnest of the White House shot back later, saying: "I think it is difficult to have a lot of patience for an American CEO trying to execute a complicated financial transaction to avoid paying taxes in America, talking about what it means to be a good citizen of the United States."

In the case of Halliburton and Baker Hughes, the Justice Department argued that putting the two together would shrink the business of providing equipment and services to oil drillers to essentially two companies, Halliburton and Schlumberger. Baer declared the merger "unfixable."

In a statement, Halliburton and Baker Hughes said that they would fight the lawsuit and would show that the oil field services business would remain competitive if their merger were completed.

Reaching into the board room

Some are questioning the administration's aggressiveness on deals.

"This is a classic case where the government seems to be overreaching into the board room," said Charles Murphy, a professor at the Stern School of Business at New York University and a former global head of investment banking at Credit Suisse First Boston.

Still, bankers are expected to lose out on hundreds of millions in fees that they would have earned had the Pfizer and Allergan deal closed.

"Here's one of the last parts of the banking business that's actually working and these guys seem to be taking the baton out and hitting them over the head," Murphy said.

The New York Times