The shortest war in history lasted 38 minutes.
On 27 August 1896 Britain demanded that Sultan Khalid bin Barghash step down. At 09:02 five warships opened fire on the palace at Zanzibar; by roughly 09:40 it was over. Britain had one wounded sailor. On the Zanzibari side around 500 people were killed or injured.
Useful: Short does not mean harmless – casualty numbers say more about a conflict than its duration.
Source: en.wikipedia.org