Would U.S. authorities want to make such a case? The historical record barely 30 cases in 225 years pronounces a firm "no."
The last person convicted of treason was Tomoya Kawakita, a Japanese-American sentenced to death in 1952 for tormenting American prisoners of war during World War II. Even such a clear-cut case created qualms; President Eisenhower commuted Kawakita's sentence to life imprisonment.
The difficulties of meeting the tough constitutional standards two witnesses or a confession in court to "levying war" against the United States help explain why treason is rarely prosecuted.
No one at Aaron Burr's 1807 trial doubted that he wanted to make himself emperor of Mexico, and probably part of the United States. It's just that planning war was not the same as "levying war," said Chief Justice John Marshall, who acquitted Burr.
Even when convictions are won, they can come back to haunt the authorities. Convicting propagandist "Tokyo Rose" in 1949 was easy enough. When it emerged years later that she had remained a loyal U.S. citizen who secretly plotted against her Japanese masters, an apologetic President Ford pardoned her.
Walker could meet the two-witness standard if he was seen bearing his AK-47 rifle on the side of the Taliban.
Proving whether that constituted "levying war" is another story. Walker joined the Taliban in March, before anyone had any idea the sides would be at war. He could argue that to desert after Sept. 11 would have guaranteed his death.
The government also would have to prove that Walker's commanders made clear to him he was fighting Americans, said John Burris, a prominent California civil rights lawyer.
"He could easily have thought he was defending the Taliban against internal enemies," said Burris.
Walker could argue that Congress has not formally declared war, although that would likely fail, said Sean Murphy, a law professor at George Washington University.
"We have the resolution by the president, authorized by both houses of Congress," he said.
U.S. authorities, who say Walker is now being helpful, have not announced their plans for the 20-year-old former Fairfax, Calif., resident. On Friday, he was moved from a forward operating base in southern Afghanistan to a Navy ship in the Arabian Sea.
He's not about to walk away, though.
"History has not looked kindly upon those who have forsaken their countries to go and fight against their countries," Attorney General John Ashcroft told senators last week.
Robert Turner, a law professor at the University of Virginia, said a likelier charge would be to link Walker to the killing of a federal official. Walker was being interrogated by CIA officer Mike Spann just minutes before Spann was killed during a prison uprising in which Walker was shot in the leg.
"The government would have case, but Walker's not a good candidate for treason," Turner said. "You use it for important players, not foot soldiers."
There is no shortage of Americans who have taken up arms for the other side, from the Revolution through the last century's world wars and the Cold War. But the reluctance to try people for their political beliefs, however obnoxious, is a mainstay.
"An offense that is inherently political has always been in deep tension with our founding principles of political freedom," said Peter Spiro of New York's Hofstra Law School.
The framers of the Constitution crafted the narrowest definition of treason known until that time because they sought to protect Americans from the blunt instrument that European rulers had used against political enemies.
"Each of them had committed treason just 12 years before," noted John Hughes, a political science professor at Saint Michael's College in Vermont.
Instead, prosecutors are more likely to bring charges that tend less to wander into thorny political arguments: espionage, sedition and sabotage.
Most Confederate leaders were granted amnesty after the Civil War. John Wilkes Booth's co-conspirators in the assassination of President Lincoln were charged not with treason, but with criminal conspiracy to aid the rebellion and with murder.
Several of the eight German saboteurs caught on these shores during World War II were U.S. citizens, but prosecutors preferred a military tribunal to a less predictable civilian jury. All eight were convicted, and six were executed.
Prosecutors charged espionage when they tried Julius and Ethel Rosenberg in 1951 with passing nuclear secrets to the Soviets. Treason, with its two-witness rule, would likely have failed. The case rested mostly on a single witness, Ethel's brother.
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