A non-aggression pact is an international treaty between two or more states, agreeing to avoid war or armed conflict between them even if they find themselves fighting third countries, or even if one is fighting allies of the other.
The most famous is the 1939 Molotov-Ribbentrop Pact between the Soviet Union and Germany, which lasted until the 1941 German invasion of the Soviet Union in Operation Barbarossa. They promised not to go to war with one another. While the rest of the shocked world tried to figure this out, Hitler attacked Poland. Britain stepped in to honour their allegance to Poland, and gave Hitler an ultimatum: If he did not withdraw in the next two days, Britain would declare war on Germany.
Examples of such pacts in history:
