Article III
Section. 3.
Clause 1: 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.
Waging war against "We The People" is the highest form of treason.
...and it is not dependent upon the use of foreign troops.