WASHINGTON — The Pentagon has provided Congress with its first detailed list of military options to stem the bloody civil war in Syria, suggesting that a campaign to tilt the balance from President Bashar al-Assad to the opposition would be a vast undertaking, costing billions of dollars, and could backfire on the United States.

The list of options — laid out in a letter from the chairman of the Joint Chiefs of Staff, Gen. Martin E. Dempsey, to the chairman of the Senate Armed Services Committee, Carl Levin of Michigan — was the first time the military has explicitly described what it sees as the formidable challenge of intervening in the war.

It came as the White House, which has limited its military involvement to supplying the rebels with small arms and other weaponry, has begun implicitly acknowledging that Mr. Assad may not be forced out of power anytime soon.

The options, which range from training opposition troops to conducting airstrikes and enforcing a no-fly zone over Syria, are not new. But General Dempsey provided details about the logistics and the costs of each. He noted that long-range strikes on the Syrian government’s military targets would require “hundreds of aircraft, ships, submarines and other enablers,” and cost “in the billions.”