How about the Franco-Prussian war ?
Prussian premier Bismarck provoked Napoleon III by publishing a (heavily edited) version of the Prussian king Wilhelm’s conversations with the French ambassador. The topic of the conversations: Problems concerning the successor to the Spanish throne. What is more, Prussia and France basically agreed on the subject, and king Wilhelm didn’t want to go to war - he was at a spa in Baden-Baden when hostilities started.
Napoleon III took the bait, declared war and got his proverbial butt kicked. When the smoke had cleared, France had lost Alsace-Lorraine (Elsass-Lothringen) and Germany had been unified under king (now emperor) Wilhelm.
Hmmm - on second thoughts, this war probably only counts as dumb from the French point of view: Bismarck got exactly what he wanted.