Nothing so dramatically and repeatedly illustrates the law of unintended consequences as war. From Charles Hugh Smith at oftwominds.com:
Initial victories do not guarantee the war will be won. Rather, they arouse the most dangerous enemy: the fatal hubris of over-confidence.
War tops the long list of human folly for a basic reason: it rarely achieves the initial goals of launching the war. It takes a special kind of human folly to discount all the negative possibilities that come from starting a war and focus exclusively on the one positive outcome in the belief it is inevitable, guaranteed, etc.
Wars carry a particularly heavy curse, that of long-term second order effects.
The decision to launch a war must discount bad outcomes and extrapolate previous minor military campaigns as “proof” that the war will be won quickly and with minimal second order effects. (First order effects: actions have consequences. Second order effects: consequences have consequences.) Put these two gratifying assumptions together and you arrive at a third assumption: the war will be over before we know it.
And so civilians make haste to view the initial battle lest they miss the all-too-brief excitement (First Battle of Manassas, American Civil War) or the combatants proclaim the war that started in late August will be over by Christmas (World War I). Alas, both wars dragged on for over four years as the bodies and consequences piled up.
All sorts of contingencies arise in war as plans go awry. For example, supplies viewed as more than ample for the expected lightning war run out as the war drags on, and there were no plans to resupply during the conflict. Opponents who were expected to run out of defensive ordnance manage to get resupplied, often by ingenious methods the attackers overlooked in their haste to grasp the easy, quick victory.