Peacekeeping operations occur mainly in what are called, failed states, and its generally a lost cause in the long run. Thats because the failed states are nations that lack the political resources to keep a functioning government going. The usual reason for this failure is a combination of ethnic and religious animosities, plus corruption and inept leadership. Several organizations (including the CIA) compile lists of the failed states, and, in the end, it appears that about a third of the worlds population live under the misrule (or no rule at all) of these nonfunctional governments. All the analysts agree that the worst of the failed states include Chad, Congo, Haiti, Iraq, Ivory Coast, Liberia, Sierra Leone, Somalia, Sudan and Yemen. Not all of these (like Chad and Yemen) are currently in need of peacekeepers, but its expected that will change in the next few years. Chad and Yemen have had long periods of civil war and general disorder in the past few decades.
Failed states have been around for a long time. In fact, one of the major achievements of mankind was the development of the nation state. But as the history of the last few thousand years has made clear, creating a nation state is difficult, and running it efficiently is even more of a chore. Despite the efforts of the long-gone European colonial governments, and nationalistic locals, most of Africa still remains an ungovernable place. It was that way when the Europeans showed up in force during the 19th century. Back then, most of the continent had no government beyond that provided by tribal councils, or more ambitious ethnic warlords. It had been that way since the dawn of time. It was still that way in many other parts of the world.
The establishment of the United Nations after World War II led to the generally accepted fiction that the entire planet was run by real, functioning, governments. As we enter the 21st century, its become obvious that large areas still lack functioning governments. And many that do currently have a government, are cursed with problems that threaten to destroy those governments at any minute. A classic case was Ivory Coast, which for a long time was seen as one of the best governed nations in Africa. But the key to the stability in Ivory Coast was local politicians staying away from ethnic or tribal demagoguery. Eventually, one major politician played the ethnic card, and chaos followed. Same situation in Iraq, where Kurds, and Sunni and Shia Arabs, were kept in line for centuries by the domination of the Sunni Arabs (about 20 percent of the population, but owner of nearly a hundred percent of the guns). The Iraq situation is not unique. Many other, seemingly peaceful, parts of the world remain that way only because one minority terrorizes the others into subservience.
Peacekeeping can bring temporary peace to an area, although sometimes even that is not possible. But the UN has made it an institutional imperative to at least try. Most of the tries have been expensive failures, but this has at least focused attention on the underlying problems that create the failed states. Its not the colonial legacy (in many of these hell holes, some locals have asked for the colonial powers to come back), or imperialism, but local problems that demand solutions from the locals themselves. Unfortunately, the locals too often opt for ethnic cleansing or a police state as the most workable solution. The peacekeeping community is still working on that.