Soviet Russia, of course, signed a non-aggression pact with Nazi Germany at the beginning of World War II only to have the latter terminate the treaty with extreme prejudice with Operation Barbarossa. Thereupon, the Russians became best buddies with the U.S. and U.K. The following is a perhaps apocryphal anecdote of the extent and suddenness of the change. The occasion was the visit of a British delegation, including Prime Minister Churchill, to Moscow in September 1941:
*Winston S. Churchill, The Second World War, vol. 3 "The Grand Alliance," (374).