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 1939Molotov-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.
The Avalon Project : Nazi-Soviet Relations 1939-1941: Treaty of Nonaggression Between Germany and the Union of Soviet Socialist Republics.
Treaty of Nonaggression Between Germany and the Union of Soviet Socialist Republics
ARTICLE VI The present treaty is concluded for a period of ten years, with the provision that, in so far as one of the High Contracting Parties does not denounce it one year prior to the expiration of this period, the validity of this treaty shall automatically be extended for another five years.
The Molotov-Ribbentrop Pact, also known as the Hitler-Stalin Pact or Ribbentrop-Molotov Pact or Nazi-Soviet Pact and formally known as the Treaty of Nonaggression between Germany and the Union of Soviet Socialist Republics, was a non-aggression treaty between the German Third Reich and the Soviet Union.
Although officially labeled a "non-aggression treaty", the pact included a secret protocol, in which the independent countries of Finland, Estonia, Latvia, Lithuania, Poland and Romania were divided into spheres of interest of the parties.
As the French had not honoured their 1924 treaty with the Czechs, the Soviets concluded that their 1935 alliance with France was valueless, and that the West was trying to divert Germany to the East.