Quisling

[Kai]

A word Norwegians are not very proud of having given to the world: it derives from [Vidkun] [Quisling] (1887-1945), a Norwegian politician who collaborated with the Nazis during World War II. He established his name as a synonym for traitor, someone who collaborates with the invaders of his country, especially by serving in a [puppet government].
It is, arguably, the worst and most grave insult a Norwegian can throw at another.

[Kai]

A traitor who collaborates with an enemy force occupying their country.

[Sonia]

a person who helps an enemy that has taken control of his or her country