The key distinction between Petraeus and Allen: Sources say Petraeus’s affair with Paula Broadwell began after he retired from the Army while Allen remains on active duty.

Although adultery is rarely a criminal matter in the civilian world, it is formally barred under the Uniform Code of Military Justice, to which Allen remains subject even as the senior four-star commander of U.S. and international troops in Afghanistan. If the DoD inspector general’s report concludes Allen violated the UCMJ, it would present his immediate boss, Gen. James Mattis of Central Command — and Pentagon officials in Washington — with what could be some tough choices.

Should they let Allen, 58, just retire from the Marine Corps or should they prosecute him? If he is cleared and wants to stay on duty, should the president continue to support his taking over the post in charge of European Command? That would require confirmation by the Senate — Allen will not take part in what would have been his confirmation hearing Thursday — and it could become a circus.