A letter from the CEO of Aetna sent to the Department of Justice in July warned that the government's opposition to an attempted merger would cause Aetna to pull out of Obamacare.

The threat went something like: Aetna's merger with fellow healthcare giant Humana is necessary for its sustainable growth, and its sustainable growth is necessary to participate in the Obamacare state exchanges. In other words: Nice healthcare exchange you got there, would be a shame if something were to happen to it.

The letter, obtained by the Huffington Post through a Freedom of Information Act request, was in response to the DOJ asking if the outcome of the attempted deal would affect Aetna's participation in Obamacare. The answer was, a lot.

"[I]f the DOJ sues to enjoin the transaction, we will immediately take action to reduce our 2017 exchange footprint," CEO Mark Bertolini wrote. "[I]nstead of expanding to 20 states next year, we would reduce our presence to no more than 10 states."

"[I]t is very likely that we would need to leave the public exchange business entirely and plan for additional business efficiencies should our deal ultimately be blocked," he went on. "By contrast, if the deal proceeds without the diverted time and energy associated with litigation, we would explore how to devote a portion of the additional synergies ... to supporting even more public exchange coverage over the next few years."

Only a few months ago, Bertolini told stockholders that "we see this as a good investment."

Before Bertolini's letter was published on Wednesday, critics such as Sen. Elizabeth Warren had openly speculated that Aetna's plan to withdraw from Obamacare was retaliation for the DOJ's suit against its merger.