A non-aggression pact or neutrality pact is a national treaty between two or more states/countries where the signatories promise not to engage in military action against each other.
Sometimes non-aggression pacts and neutrality pacts have been considered different. In that case non-aggression pact is considered to include the promise not to attack the other signatories, while a neutrality pact includes a promise to avoid any support against the other signatories. In 19th century neutrality pacts have often been used to give permission to attack another state.
It was a popular form of international agreement in the 1920s and 1930s, but has largely fallen out of use after the Second World War. Since the implementation of a non-aggression pact depends on the good faith of the parties, the international community following the Second World War adopted the norm of multilateral collective security agreements, such as the treaties establishing NATO, ANZUS, SEATO and Warsaw Pact.
An example of non-aggression pact is the 1939 Molotov–Ribbentrop Pact between the Soviet Union and Nazi Germany, which lasted until the 1941 German invasion of the Soviet Union in Operation Barbarossa.
It has been found that major powers are more likely to start military conflicts against their partners in non-aggression pacts than against states that do not have any sort of alliance with them.
Aggression is forbidden by the UN Charter, so all UN member nations are under a treaty obligation not to commit any acts of aggression (of course this has often been broken in practice).