Before the war. This comes directly from the Latin words ante and bellum, meaning 'before the war'. Pronounced antee-belem.
In the USA, particularly in the south, this is used to refer to before the American civil war. It began to be used in this specific sense almost as soon as the American civil war began. Somewhere around 1905 the words prewar and postwar entered the English language, so antebellum was not applied to the world wars.
In the UK (and other areas) antebellum can be used to refer to any war, particularly the Boer War or either of the World Wars.