Section 3. Treason against the United States, shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort. No person shall be convicted of treason unless on the testimony of two witnesses to the same overt act, or on confession in open court.
The Congress shall have power to declare the punishment of treason,
U.S. Code Definition:
Whoever, owing allegiance to the United States, levies war against them or adheres to their enemies, giving them aid and comfort within the United States or elsewhere, is guilty of treason and shall suffer death, or shall be imprisoned not less than five years and fined under this title but not less than $10,000; and shall be incapable of holding any office under the United States.
Case law on the meaning of "levying war" against the United States focuses on very early affairs in the Nation's history, such as Aaron Burr's scheming. Chief Justice Marshall and company laid out a requirement that there be a conspiracy, "an assemblage" of men. But in Ex parte Bollman and Swartout the Court was focused on the Burr-related facts at hand, where the attack did not fully unfold:
the Chief Justice had said that it was not the court’s intention “to say that no individual can be guilty” of treason “who has not appeared in arms against his country.”
so it's unclear to me whether the conduct of the Fort Hood gunman could fit within the definition of levying war against the United States as...an"assemblage" or a "body" of one. Bollman focuses mostly on the distinction between plotting to levy war and actually levying war, but the case cites an earlier case by Judge Chase approvingly, though, noting:
they are guilty of the treason of levying war; and the quantum of the force employed, neither lessens nor increases the crime: whether by one hundred, or one thousand persons, is wholly immaterial.'
While it appears he was acting alone, the gunman's attack - as a U.S. military officer killing and injuring a significant number of fellow military personnel on a military base while the U.S. is engaged in conflicts overseas - seems to rise to the level of heinousness that the treason charge is intended to capture. It involves the wickedness and betrayal of turning on one's fellow soldiers, even if the gunman did not realistically believe he would overthrow the U.S. government, another focus of some of the early cases.
My heart goes out to the families of those killed in the attack at Ford Hood. What a despicable thing.