A side, party (e.g. a team or armed force) seen as adversary, enemy etc. opposite one (or more) other(s), as in a battle or team sports competition.
Someone who takes arms and levies war against their country; or one who aids an enemy in conquering their country.
One who betrays any confidence or trust.
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 their defense, or surrenders an army or body of troops to the enemy, unless when vanquished.
Traitorous.
To act the traitor toward; to betray; to deceive.