The shortest recorded war in history took place on 27 August 1896, between the British Empire and the Sultanate of Zanzibar. It was Britains action against the new Sultan who replaced a ruler far more sympathetic to the British, the war lasted for around 40 mins (maximum 45 depending on which ships logbook you believe). The British surrounded the palace with war ships and soldiers and unleashed a barrage of attacks until the palace barely stood, the Sultan escaped into exile leaving his subjects to carry on the defence of his rule.

http://en.wikipedia.org/wiki/Anglo-Zanzibar_War