Responding to questions about the Syrian crisis from Senate Armed Services Chairman Carl Levin (D-Mich.) and Sen. John McCain (R-Ariz.), Dempsey said the Pentagon was prepared for at least five options: train, advise and assist the opposition; limited stand-off strikes; a no-fly zone; buffer zones; and controlling chemical weapons.

But, Dempsey warned bluntly, each would be risky and expensive — some more than others — with projected costs of several options at $1 billion a month.

“All of these options would likely further the narrow military objective of helping the opposition and placing more pressure on the regime,” Dempsey told Levin in a three-page letter dated Friday and released Monday by the senator.

But the four-star Army general warned: “It is not enough to simply alter the balance of military power without careful consideration of what is necessary in order to preserve a functioning state. We must anticipate and be prepared for the unintended consequences of our action. Should the regime’s institutions collapse in the absence of a viable opposition, we could inadvertently empower extremists or unleash the very chemical weapons we seek to control.”

The use of such force is “no less than an act of war,” he said, and would come at a time of growing fiscal restraint on the Pentagon.

“Some options may not be feasible in time or cost without compromising our security elsewhere,” he said.

Levin and McCain requested more information from Dempsey on the use of military force in Syria last week as well as the path forward in Afghanistan in the wake of his reconfirmation hearing as chairman of the Joint Chiefs of Staff.

During the hearing, McCain tangled with the general over Syria and afterward vowed to block his renomination in the Senate until he got answers to his questions.

On Monday, a McCain aide told POLITICO Dempsey’s letter released by Levin was in response to a request from Levin during the course of the hearing, rather than to the questions the two senators posed jointly afterward.

In his letter, Dempsey said he preferred a “regional approach” to the Syrian conflict that would isolate it “to prevent regional destabilization and weapons proliferation.”

“At the same time,” he said, “we should help develop a moderate opposition, including their military capabilities, while maintaining pressure on the [Bashar] Assad regime.

“Too often, these options are considered in isolation,” Dempsey said. “It would be better if they were assessed and discussed in the context of an overall whole-of-government strategy for achieving our policy objectives in coordination with our allies and partners.”