It's simple to start a war, common to win battles, but difficult and rare to complete a war with one's objectives achieved.
This crucial final step ought to be about as simple as the second according to Clausewitzian military theory: All you need to do is win big enough or many enough battles to 'disarm' your opponent (to deny your opponent the ability to resist). A disarmed power yields to your will--that's what the theory implies.
There are several exceptions to this rule which turned all-too many conflicts even messier than anticipated. One such exception has gained a lot of attention in the last about ten years: The opponent could devolve into a lesser state of organization and persist (as a guerilla force, for example). The opposing power might even avoid disarmament by becoming elusive and by keeping the intensity of warfare at a level which doesn't exceed its ability to regenerate its potential for violence: A conflict cooled down just enough to sustain the refusal of offered conditions.
There's a different and historically very powerful case; some wars are fought over a distance which doesn't allow the initially-superior power to force a decision. The despair of non-nomadic invaders of Russia comes to mind. Imperial Japan faced a similar difficulty in its war planning. It did defeat Russia in 1905, but probably only because Russia was politically unstable and at the brink of a thorough revolution. This kept Russia from continuing the land war with a hastened completion of the Trans-Siberian Railway.
Later on, Imperial Japan faced the dilemma that it couldn't possibly bring the US, UK or France to their knees. Even the loss of Singapore was but a small scratch inflicted on the British Empire, and even the British shipyards in isolation would have been able to compensate for a lost “decisive” battle within three to four years. The French could have ignored the loss of Indochina and French Polynesia for decades without agreeing to peace. The US could have ignored the loss of the Philippines and rebuild its fleet in three-year intervals completely.
A Mahanian focus on decisive naval battles, reinforced by the memory of the Battle in the Tsushima Strait, was the Japanese' mental escape from this Sword of Damocles. The Clausewitzian view treats a major victory in a battle of great army concentration (and by theoretical extension, its naval equivalent) as disarming and war-winning because this was true for the relatively small and neighboring powers in Europe. The stubbornness of the people of Spain under the Napoleonic occupation and the ability of the Russians to sacrifice vast areas of land including their biggest city without yielding should have signaled the very limited validity of this view from the very beginning.
The difficulty in reaching a satisfactory completion of war by defeating the enemy's military might coined the 20th century: The British Empire refused to accept defeat because, though inferior on land and in the air, it was able to avert an invasion of England. The asymmetry between a land-centric power and a naval-centric power precluded the Clausewitzian decisive clash of Schwerpunkt forces vs. Schwerpunkt forces and thus a Clausewitzian completion of the war.
Guerillas all over the world followed the Spanish example of averting final defeat and survived as political movements even if temporarily disarmed, rarely ceasing resistance entirely.
And then there's another completion of war, without a decisive battle (though some scholars will stretch the meaning of Schwerpunkt beyond recognition to cover this case): The strategic coup de main, which often precluded a major war with its fait accompli: Often times it's simply not worth or promising enough to wage war when the other power has already grabbed what it wanted.
The Shiites and Sunni of Iraq dealt the real decision of the recent Iraq war by the fait accompli of ethnic cleansing and majority rule, while Americans were being fed stories about harassing attacks with mines that had no real bearing on the outcome:
Such a demographic change will likely last for centuries, whereas the question whether the harassment of convoys with the mine campaign was defeated or not is inconsequential in Iraq today already.
The completion of war after a fait accompli is typically found once some face-saving exit is being left open by the "winner" or created by the loser through sheer narrative manipulation. The recent conflict about the Crimea shows the power of the coup de main and its achievement of a fait accompli: The Ukrainian military wasn't disarmed or incapable of continued resistance; it hadn't even completed gearing up by the time the Ukraine de facto accepted the loss of the Crimea to Russia since reconquest or intervention of the UNSC was out of question.
It is notable that much of the (largely unsuccessful) Western interventionism, such as cruise missile diplomacy, bombings, assassination drone campaigns, military assistance programs and no-fly zones was devoid of a decisive battle, fait accompli or offering the enemy a face-saving exit.
It's no wonder Western scholars of military affairs are bemoaning the difficulty of “successfully” completing a war: The West is thoroughly incompetent at it, while others aren't.
This is something even gold-plated combat aircraft, multi-billion dollar warships, nuclear weapons, UNSC veto powers and the heaviest infantry of the world cannot change.
Sven Ortmann is a German blogger. Since 2007, his blog, “Defence and Freedom,” has covered a range of military, defence policy and economic topics, with more than a million page views. His personal military background is his service in the Luftwaffe. He has guest-blogged at the Small Wars Journal Blog and other blogs on military topics.