Protracted Warfare

There is an important lesson our leaders have failed to learn about warfare. It is a lesson known to all successful conquerors, but not to those who lead empires to destruction. It is often the cause of the fall of their empire.

Protracted warfare is not war at all. It is training for your enemies.

The Spartans were the most powerful of all the Greek city-states, and defeated the up-and-coming Athenians in the Pelopponesian War. But they were eventually upset by the Thebans. This quote explains how that came about.

[Agesilaus] waived the exemption by law which he had formerly claimed in the matter of the expedition, and presently led an incursion into Boeotia himself, where he inflicted damage upon the Thebans, and in his turn met with reverses, so that one day when he was wounded, Antalcidas said to him: "Indeed, this is a fine tuition-fee which thou art getting from the Thebans, for teaching them how to fight when they did not wish to do it, and did not even know how." For the Thebans are said to have been really more warlike at this time than ever before, owing to the many expeditions which the Lacedaemonians made against them, by which they were virtually schooled in arms. And Lycurgus of old, in one of his three so‑called "rhetras," forbade his people to make frequent expeditions against the same foes, in order that those foes might not learn how to make war.  -- Plutarch, Life of Agesilaus

Sun Tzu warns of the dangers of prolonged warfare in "The Art of War".

There is no instance of a country having benefited from prolonged warfare. 

Contributing to maintain an army at a distance causes the people to be impoverished.
On the other hand, the proximity of an army causes prices to go up; and high prices cause the people's substance to be drained away.
When their substance is drained away, the peasantry will be afflicted by heavy exactions.
With this loss of substance and exhaustion of strength, the homes of the people will be stripped bare, and three-tenths of their income will be dissipated; while government expenses for broken chariots, worn-out horses, breast-plates and helmets, bows and arrows, spears and shields, protective mantles, draught-oxen and heavy wagons, will amount to four-tenths of its total revenue.

When you engage in actual fighting, if victory is long in coming, then men's weapons will grow dull and their ardor will be damped. If you lay siege to a town, you will exhaust your strength.
Again, if the campaign is protracted, the resources of the State will not be equal to the strain.
Now, when your weapons are dulled, your ardor damped, your strength exhausted and your treasure spent, other chieftains will spring up to take advantage of your extremity. Then no man, however wise, will be able to avert the consequences that must ensue.

-- Sun Tzu, The Art of War, book 2 

Mao Tse-tung also understood the dangers of protracted warfare, but he used them to his advantage when he was in a weak position. He espoused protracted warfare because he needed time to build up his strength against the stronger Japanese and Chinese Nationalist forces. And when he was strong enough, he went for the quick, decisive victory.

Russia fought a 10 year war in Afghanistan, which they were unable to win. This was demoralizing to the Soviet army.

Due to its length it has sometimes been referred to as the "Soviet Union's Vietnam War" or the "Bear Trap" by the Western media, and thought to be a contributing factor to the fall of the Soviet Union. -- Wikipedia, Soviet–Afghan War

The United States is now involved in a war in Afghanistan that started in 2001 and a war in Iraq and Syria which started in 2003. Wars of this length are always harmful, if not outright disastrous, to the empire who attempts to wage them. We are training our enemies, ruining our own economy, and patiently waiting for the consequences that must ensue.