The top of the health insurance industry is poised to become an even more exclusive club. In recent weeks, news has broken that the nation’s five largest health insurance companies (UnitedHealth, Anthem, Aetna, Cigna, and Humana) are jostling to acquire a major competitor or be acquired by one. It appears that it is only a matter of time before another round of health insurance industry consolidation commences, potentially raising anti-trust concerns.

Over the weekend, Anthem announced plans to acquire Cigna in an attempt to put pressure on the smaller company’s leadership. Discussions regarding a merger between Anthem and Cigna dated back to last year, and Anthem privately made four buyout offers since June 4. Cigna’s rejection of these offers, the last of which represented a 35% premium over Cigna’s stock price on May 28, prompted Anthem to go public with its $54 billion proposal.

Anthem is currently a distant second to market leader UnitedHealth, with $73.9 billion in annual revenue to UnitedHealth’s $130.5 billion. Cigna brought in $34.9 billion last year, and Anthem believes a merger can produce $2 billion in synergies. Thus if the merger is consummated and approved by regulators, Anthem will make significant progress in closing the gap with its chief rival.

In a letter, Anthem CEO Joseph Swedish indicated that the Cigna Board was holding up the deal because Anthem would not guarantee that Cigna’s CEO, Joseph Cordani, would assume that position in the merged company. Analysts believe that Anthem’s open letter is intended to convince Cigna shareholders that its leadership is missing an opportunity to increase shareholder value because they are prioritizing their personal interests above those of Cigna shareholders.

Cigna responded to the letter with one of their own directed to Anthem’s board. Cigna stated that it rejected Anthem’s acquisition not out of personal pride, but because it had doubts about Anthem’s growth and marketing strategies. Further, analysts have noted that Anthem’s proposal would increase its capital-to-debt ratio from 39% to 50%, a figure that “rather unprecedented” for a major health insurer. Market watchers have also speculated that Cigna could be weighing Anthem’s proposal against the alternative of making its own acquisition of Humana, which is also being courted by Aetna.

As the reigning market leader, UnitedHealth group is not thought to be under pressure to make an acquisition at this time. However, it has also been mentioned as a possible partner for a company that could be left out of the current merger frenzy. As Fortune magazine noted, if UnitedHealth and Aetna merged, the company would rank fifth on the Fortune 500, becoming the world’s largest healthcare company. Perhaps coincidentally, UnitedHealth announced yesterday that it will leave the health insurance industry’s largest trade group, America’s Health Insurance Plans (AHIP), at the end of June because it believes the group no longer represents the company’s interests.

While it is unclear how this high-stakes game of musical chairs will end, what is certain is that more change is coming to the industry, this time from market forces rather than government regulation.