When is war justified?

After World War One there was a great concern among the society how to keep the peace that was established in Versailles in June 1919. To ban war as a means of achieving political aims, at the request of France, a Briand- Kellogg Pact was signed (by 1939 by 75 countries) in 1928. This was to ensure that only diplomatic ways can be used to achieve a country’s aims, but it was not obeyed (only once it came in as a ground for a peaceful settlement between the USSR and China over the Manchurian railway in 1929).
There were strong pressures on the governments of the European countries which lead to the popularity of the ‘appeasement policy’, which proved to be faulty. The First World War threw Europe into instability, all economic, social and political. When Adolf Hitler, who came to power in 1933, revised the Treaty of Versailles, ignoring its assumptions (by, for example, bringing back the German army to the demilitarized zone of the Rhineland), the Great Britain and France, inste...