One who violates his allegiance and betrays his country; one guilty of treason; one who, in breach of trust, delivers his country to an enemy, or yields up any fort or place intrusted to his defense, or surrenders an army or body of troops to the enemy, unless when vanquished; also, one who takes arms and levies war against his country; or one who aids an enemy in conquering his country. See Treason.
Hence, one who betrays any confidence or trust; a betrayer.
A traitor is a betrayer - one who practices injury, while professing friendship. Benedict Arnold was a traitor, solely because, while professing friendship for the American cause, he attempted to injure it. An open enemy, however criminal in other respects, is no traitor. Lysander Spooner
Millions of Americans cannot tell you who lived at Mount Vernon or who wrote the Declaration of Independence - let alone the Emancipation Proclamation. But they know that to be 'a Benedict Arnold' is to be a traitor of the deepest dye - someone who coldly betrays not only a sacred cause but every moral scruple along the way. Arthur L. Herman