Operation Tannenbaum, known earlier as Operation Grün ("Green"), was a planned invasion of Switzerland and Liechtenstein by the Axis Powers during World War II.

During World War I and World War II, Switzerland maintained armed neutrality, and was not invaded by its neighbors, in part because of its topography, much of which is mountainous. Germany was a threat and Switzerland built a powerful defense. It served as a "protecting power" for the belligerents of both sides, with a special role in helping prisoners of war. The belligerent states made it the scene for diplomacy, espionage, and commerce, as well as being a safe haven for 300,000 refugees.