Someone who betrays his country by committing treason WordNet 3.0 & Open English WordNet

Someone who violates an allegiance and betrays their country; someone guilty of treason; one who, in breach of trust, delivers their country to an enemy, or yields up any fort or place entrusted to his defense, or surrenders an army or body of troops to the enemy, unless when vanquished. Wiktionary

One who violates allegiance and betrays one's country. Wiktionary (translation)

One who commits treason. Wiktionary