What was the status of war under legal norms in traditional International Law before World War-I ?

Traditional international law before the First World War placed no general ban on resort to war. War was treated as a sovereign prerogative and a political fact, outside legal regulation as to its legality, though its conduct was governed by the laws of war.

War was not made illegal until the Covenant of the League of Nations, the Kellogg-Briand Pact of 1928 and the UN Charter of 1945, so "UN approval" could not have been a requirement before 1914. No rule required all States to join a war.