Tonight Fox allowed a guest to perpetuate a debunked myth, born on right-wing blogs, that U.S. Marines at the U.S. embassy in Cairo had been banned from carrying live ammunition, even though hours earlier the Marine Corps had dispelled that rumor.

This morning, conservative national security blog Night Watch began hyping a rumor that U.S. Ambassador to Egypt Anne Patterson had banned Marines protecting the embassy in Cairo from carrying live ammunition. The unconfirmed report was quickly picked up by right-wing blogs, including Breitbart.com, The Washington Free Beacon, and Glenn Beck's TheBlaze. (As of this writing, only The Washington Free Beacon had corrected its story.)

In response, the U.S Marine Corps discredited the rumor, calling it “not accurate.” From the Corps congressional liaison's memo:

The Ambassador did not impose restrictions on weapons or weapons status on the Marine Corps Embassy Security Group (MCESG) detachment. The MCESG Marines in Cairo were allowed to have live ammunition in their weapons. The Ambassador and Regional Security Officer have been completely and appropriately engaged with the security situation. Reports of Marines not being able to have their weapons loaded per direction from the Ambassador are not accurate.

Additionally, as Mother Jones points out, a glance at the State Department's guidelines reveals that an ambassador could not give such an order. Accordingly to State Department regulations, Marines may be assigned “duties other than those previously described in this section to the Marines as may be required by urgent or security-related circumstances requiring immediate action,” but "[s]uch duties shall not contravene established Department or Marine Corps policy and shall not unduly jeopardize the safety or well-being of any Marine."

And yet, retired Lt. Col. Bob Maginnis repeated the discredited rumor as fact while appearing on Fox Business' Lou Dobbs Tonight. While discussing security at U.S. embassies and consulates in the Middle East, Maginnis declared that in Cairo, “supposedly ... the ambassador told the Marines not to take live ammunition out into the yard” while protestors scaled the embassy walls. Maginnis even implied that Patterson's supposed ban on live ammunition contributed to the violence, saying: “That type of thinking is dumb in that part of the world. It's asking for it.” Host Lou Dobbs failed to challenge any of Maginnis' claims.

Allowing uncritical repetition of a debunked rumor is in keeping with Fox's effort -- and that of other right-wing media figures -- to cast blame on the Obama administration for the attacks on the Cairo embassy and Libyan consulate.