Let us learn our lessons. Never, never, never believe any war will be smooth and easy, or that anyone who embarks on the strange voyage can measure the tides and hurricanes he will encounter. The statesman who yields to war fever must realise that once the signal is given, he is no longer the master of policy but the slave of unforeseeable and uncontrollable events. Antiquated War Offices, weak, incompetent, or arrogant Commanders, untrustworthy allies, hostile neutrals, malignant Fortune, ugly surprises, awful miscalculations — all take their seats at the Council Board on the morrow of a declaration of war. Always remember, however sure you are that you could easily win, that there would not be a war if the other man did not think he also had a chance.

Winston Churchill (1874-1965) British statesman and author

My Early Life: A Roving Commission, ch. 18 “With Buller to the Cape” (1930)
    (Source)

On his overconfidence in 1899 prior to the Boer War. See Pleve (1903).