In perhaps the pettiest reason to start a war since the Americans got a bit shirty over our borrowing of their sailors in 1812, the Crimean War (1853-1856) began as a disagreement over whether Catholic or Orthodox Christians should control the churches in the Holy Land. (Of course, it was really about whether Britain or Russia was allowed to have the bigger Empire, but we don't talk about that in polite society.)
Casus belli was when Napoleon III sent a warship into the Black Sea to politely suggest that the Ottoman Sultan Abdülmecid I (crazy name, crazy guy) acknowledge French Catholicism as the authority over Christian sites in the Holy Land. This rather annoyed Tsar Nicholas I, since up till this point the Eastern Orthodox Church had controlled those sites, and he responded by sending two corps to the banks of the Danube. Diplomatic relations soured from there, and since the British and French were leery of Russian expansionism and had troops in the area anyways, they decided they might as well pitch in on the Ottoman side.
As you'd expect from the name, most of the conflict was fought in the Crimea, with the Russian Empire on one side and an alliance of Britain, France, the Ottoman Empire, and Sardinia on the other. It was also one of the first truly "modern" wars, if large masses of men shooting each other over which toff got to call themselves Emperor of Wherever can be called "modern." Technology like the exploding artillery shell, the railroad, and the telegraph shaped the conflict, and the advent of photographic technology meant it was one of the first wars to be documented in the press. This was particularly delightful in that the great legacy of the Crimean war was the rank incompetence and general mismanagement on the part of the leadership of all sides. Something to really be proud of there. If you're going to be inept, be REALLY inept. For the citizens on the home front puffed up with patriotic fervour, this was a bit like going to see your favourite band in concert only to realise that they only sounded good thanks to auto-tune and ruthless editing.
The war ended in 1856 when the allies took the key Russian city of Sevastopol. Recognising the inevitability of defeat, Nicholas I sued for peace, and as part of the terms the Black Sea was declared neutral, Moldavia and Wallachia were made independent, and everyone more or less tried to forget the entire embarrassing debacle. It did lead to extensive calls for military reform and to nationalist movements across Eastern Europe, which doesn't quite seem worth the lives of half a million people, but what do I know?