The inaccurately-named "Thirty Years' War" left a smoking crater where the center of Europe had been. What had begun as a game of slap-tickle between the Most Catholic Habsburgs and uppity Protestant Czechs quickly spun out of control into a fratricidal conflict that sucked in most of Europe from Stockholm to Madrid. When all was said and done, the largest army on the field in Western Europe was that of the corpse-eating rats, and Europe as a whole had developed a revulsion towards war that largely reduced the activity to Kabuki-like theater for the next several centuries.
From the late 17th Century to the early 20th, 'war' to the average European was something that groups of uniformed men would engage in at agreed upon times in open fields or sunny hilltops far away from un-uniformed noncombatants. (This didn't apply to shooting up Wogs, of course.) Even the revolution-era Frogs' introduction of the concept of The Nation In Arms only widened the potential pool of casualties, and not the potential theaters of war.
19th Century warning signs that war between white folk was returning to its old haunts and habits, like Vicksburg and the Prussian siege of Paris, were ignored and the conscript levees of 1914 marched towards the sound of the guns with cheery leftover Mauve Decade bonhomie. This was quickly blown apart on the fields of Ypres and Neuve Chapelle. The Germans, meanwhile, had been harassed on their march through Belgium by civilian tiralleurs and had, in their frustration, gotten a bit frisky with reprisals; from their point of view the Belgian civilians had violated the long-standing Western code of war by taking shots at the Jerries despite being dressed in mufti (we call these "illegal combatants" nowadays). Handed a propaganda victory like this, the Brits felt all the more justified in blockading German ports.
Faced with the overwhelming superiority of the Royal Navy, the Germans had only one card to play: Their U-Boat fleet. Lacking the dreadnoughts to play tit-for-tat with the RN blockade, they declared the waters around the British Isles to be a fair hunting ground for unrestricted submarine warfare. On the afternoon of May 7th, 1915, a British passenger steamer, the Lusitania, crossed the path of the U-20; the German skipper pulled the trigger on one torpedo, and the giant, almost 800-ft long, fast-moving liner settled at the bow and plowed into the ocean bottom some 300 feet below. She was carrying a bazillion rounds of .303 ammunition in her holds, which was definitely contraband and marked her as a target (although international law and convention at the time required her passengers and crew to be warned to disembark and given time to do so before she was sunk), but more importantly she was carrying 197 American passengers, of which 128 died in the wreck. The USA, possessed of a large German minority and proudly neutral until this point in what was widely seen as a European dynastic squabble, was shoved towards the Allies by what in future days would become an all too common feature of modern war: What Kapitan Schwieger did to the Lusitania, many Americans would be decorated for doing to ships with the last name "Maru" just thirty years later.