How long do civil wars last? The Spanish Civil War lasted 2 years, 8 months, 2 weeks and 1 day; the American Civil War lasted 4 years, 3 weeks and 6 days. However, the average duration for modern civil wars is about ten years. Lots of these civil wars end in a peace deal because both sides already have shot their bolt. The Syrian civil war has lasted about half that long. So far.
Why have modern civil wars dragged on for so long? Historically, foreign intervention plays a large role in prolonging civil wars. That is one reason that the Americans welcomed French support in the War for Independence and Abraham Lincoln sought to avoid British or French intervention in the American Civil War. Spain became a battle ground for Fascism (Nazi Germany and Fascist Italy) and Communism (the Soviet Union and the International Brigades raised by the Comintern). Syria has become the battle ground for radical Islam (ISIS and the Al Nusra Front); the Shi’ite side of the larger Muslim civil war (Iran, Iraq, and Syria); the Sunni side of the larger Muslim civil war (Saudi Arabia and other Gulf states); Kurdish nationalists and Turkey (which has its own issues with the Kurds); and Western powers (the USA and Russia). The multiple powers engaged only complicate a peace settlement.
Why has the Syrian civil war been so gory? Normally, say the scholars of these things, both sides in a civil war have a strong incentive to win the loyalty of the civilians who provide the “sea” in which the insurgents “swim.” This puts a check on the atrocities. It doesn’t prevent them, but it does limit them. However, the Syrian civil war is different. First, the Alawite and Christian minorities fear genocide at the hands of the Sunni majority. If you look at the broader pattern in the Middle East, this isn’t an unreasonable fear. Outside support/intervention reduces the importance of the local population in the eyes of the fighters. Thus, ISIS is OK with atrocities committed against Unbelievers, or Insufficient Believers. The government is backed by a minority of Syrians, so there is little to be gained from humane conduct toward the rebellious Sunni majority. The foreign Sunni supporters of the rebels only stand to profit from the massacre of Shi’ites. This intensifies the “normal” atrocities of war. The popular image of men with guns run amuck may not be accurate. Syria could be suffering multiple “ethnic cleansings.” The government is the “Mr. Clean” in this business, but it has competitors. Thus, many Christians and Alawite Muslims have fled to sanctuary in western Aleppo.
Is the Syrian Civil War un-winnable? This is unclear, in spite of the prognostication of the New York Times and the Obama administration.
What is the basis of a peace deal? All sides are coalitions of things that they are against, rather than things they are for. (This is much like the Russo-British-Americans alliance during the Second World War.( The Russkies want President Assad to get off the stage at some point, but aren’t—yet–willing to force him or kill him. Neither Turkey nor Iraq wants the Kurds to gain much territory or prestige. The various parties will try to hold what they have already won. (Except, perhaps, ISIS.) ISIS will be defeated, but what will become of the Sunni rebel territories? Perhaps, the country will have to be partitioned between an Assad-ruled-for-now West and an ISIS-ruled “free fire zone” in the East. Then what?
 Max Fisher, “Why Syria’s War, After 400,000 Deaths, Is Only Getting Worse,” NYT, 27 August 2016.
 This may reflect weak governments out against weak insurgencies, with lots of ordinary people caught in the middle.
 See: The Thirty Years War; see: The Treaty of Westphalia.
 More specifically, it puts a check on the actions of the psychopaths who fill the ranks of opposing armies.