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Wall Street has been cheering on a mergers and acquisitions boom all year that has been fueling investment banking revenues and investment returns for some shareholders. But on Tuesday the great M&A boom of 2014 hit something of a snag.

First, billionaire media dealmaker Rupert Murdoch walked away from his effort to buy Time Warner. Murdoch’s 21st Century Fox withdrew its $80 billion takeover offer for Time Warner after failing to draw Time Warner to the negotiating table. 21st Century Fox’s decision to instead repurchase $6 billion of its shares is sure to further frustrate its investment bankers.

Next, reports emerged that Sprint and Softbank would be giving up their effort to purchase T-Mobile in a deal that The Wall Street Journal said would have valued T-Mobile at $32 billion. The potential deal was derailed because of regulatory obstacles.

Rupert Murdoch (Photo credit: Wikipedia)

But perhaps the most serious threat to the M&A boom came from Walgreen and Washington. One of the key trends pushing the M&A boom has been inversion deals that have seen U.S. companies purchase or combine with foreign companies in ways that allow U.S. companies to domicile in foreign jurisdictions to lower their tax rates. Big hedge funds like Jana Partners and Och-Ziff Capital Management were lobbying Walgreen to do a tax inversion as part of its acquisition of Europe’s Alliance Boots. But on Tuesday it became clear that while Walgreen is planning to go ahead with its acquisition of Alliance Boots, it will do so in a way that will keep Walgreen domiciled in the U.S. Walgreen appears to have made its decision because of political pressure.

Also on Tuesday, the Treasury Department said it had started exploring ways to block inversion deals without congressional action. President Barack Obama has criticized inversion deals and wants to end them, but Wall Street has counted on congressional gridlock preventing him from doing anything about it. The Treasury Department said on Tuesday it was “reviewing a broad range of authorities for possible administrative actions that could limit the ability of companies to engage in inversions, as well as approaches that could meaningfully reduce the tax benefits after inversions take place.”

Over $2 trillion of corporate mergers and acquisitions have already been announced so far this year, making 2014 the biggest M&A year in a long time. It looked like a given that the frenzy would continue, but suddenly it seems like this M&A boom might have its limits.