With the Soviet invasion of Poland on 17 September 1939, a secret and extraordinary pact came into effect. Two opposing ideologies, National Socialism and Communism, joined forces to wipe the hated Poland off the map. Even plans for a sub-Poland were eventually dismissed. This co-operation had earlier historical roots, but was also born out of practical necessity. Besides the origins of cooperation, we will reflect on the practical implementation of this pact. Europe’s two largest armies fought the same opponent from opposite directions. With ideological diversity, conflicts and accidents lurked. Caution was imperative. Curiosity was there too; about each other’s weapons systems, each other’s armies and officers, and about the ‘Jewish’ political commissioners. Fear of friendly fire played a role a well, because accidents happen, and indeed things went wrong in several places.