Californian accused of treason

The AP reports:

he charge of treason was used for the first time in the United States' war on terrorism Wednesday, filed against a California man who appeared in propaganda videos for al-Qaida.

Adam Yehiye Gadahn, 28, could be sentenced to death if convicted of the charge, which has been used only a few dozen times in U.S. history and not at all since the World War II era.

How do you get charged with treason if there's no official declaration of war?