More than 525,000 people die each year as a result of violent conflict, and millions more are affected, according to U.N. figures.
Conflicts start for a wide range of reasons, and each conflict is different and often complex. Tensions can build up for many years over major issues, but it can take a relatively minor incident to trigger the violence.
The style of war has changed in recent decades – the majority of conflicts now take place within national borders, rather than between countries.
The Geneva Conventions which encapsulate international rules of warfare, including protection of civilians, were written when most wars were fought by armies from different nations. Now warring parties include non-state armed groups who often target civilians on a massive scale and, to a lesser extent, aid agencies. According to the United Nations Development Programme (UNDP), 90 percent of those killed in conflicts since 1990 have been civilians, many of them women and children.
Governments have two main responses to conflict: military attack and mediation. Many experts say the military option alone is rarely enough because it does not address the underlying causes of the war, and without this there can be no long-term sustainable peace.
But some wars are no longer being fought to win – they have become a means for armed groups to make money, experts say. And sometimes rebel fighters are reluctant to return to civilian life because they may struggle to make a living, or will be rejected by their home communities because of crimes they committed during the war.
Causes of war
What causes a war is a question that experts have been wrangling with for years, in the hope of finding ways to prevent or end conflict.
Every war is different, and each one often has many different causes. History, the distribution of power and access to resources can all play a part, but it is difficult to generalise.
Media coverage can oversimplify and even mask the underlying causes. A conflict may erupt over access to scarce resources or political tensions, and polarise along religious or ethnic lines. Describing it as a religious or ethnic war ignores the underlying economic, social and political factors.
Sometimes the original grievances that triggered conflicts become lost in time, and wars continue simply because there are enough people making money out of them. Armed factions want to keep control of natural resources like diamonds or timber, and combatants prefer paid fighting to unemployment and extreme poverty or fear ostracism by their home communities.
Colonialism and the Cold War have left legacies that are still being worked through today.
There remain issues over borders imposed by colonialists to suit the economic needs of European powers, which often amalgamated different nations, tribes and religious groups into one country. Colonialists also used the principle of divide-and-rule, pitting groups against each other and often openly favouring one group over another.
At the end of colonialism, groups that had been marginalised vied for greater economic and political control. And secessionists across the world tried to redefine the colonial borders, sometimes based on ethnicity or religion.
In response, many newly independent governments tried to forge some sense of national unity by heavily centralising political and economic power. Political monopolies often led to corruption, nepotism and the abuse of power, writes former U.N. Secretary-General Kofi Annan in a publication entitled "Human rights, peace and justice in Africa".
During the Cold War, superpowers Russia and the United States extended their control over many countries worldwide by supporting governments that allied with them – or militias in countries that did not – fuelling some of the world's most deadly conflicts.
When the Cold War ended, the often oppressive governments and militia groups funded and supported by the superpowers suddenly had to fend for themselves. Those governments that were weak faced violent conflict and unrest.
When the Soviet Union broke up in 1991 putting an end to the Cold War, conflicts began over borders of newly independent states and control of natural resources. Issues remain unresolved over breakaway regions – for example over Georgia's South Ossetia and Abkhazia, and over Nagorno-Karabakh disputed by Azerbaijan and Armenia – and are a source of tension in the region.
The breakup of Yugoslavia, partly because of the fall of communism in Eastern Europe in the early 1990s, was particularly brutal.
ACCESS TO POWER AND NATURAL RESOURCES
Some countries have a "winner takes all" approach to government, and the control of wealth, power and natural resources is concentrated in the hands of a few. This exacerbates tensions if political parties are either regionally or ethnically based, says Annan.
Marginalised ethnic minorities have taken up arms in many countries including Myanmar, Nigeria, Ethiopia, Sri Lanka, India, Mali, Morocco, Russia, China and Georgia.
Millions have died in resource-fuelled wars since the late 1990s. Major conflicts fuelled by revenues from diamonds, timber and minerals include Liberia, Sierra Leone, Angola, Ivory Coast, Democratic Republic of Congo and Cambodia. For more, see our briefing on resource wars.
Competition for scarce land and water in densely populated regions can raise tensions, especially in situations where large numbers of displaced people return home at the end of a war to reclaim their land.
Access to land and water is likely to worsen with climate change. Food and water shortages and migration because of climate change may increase the risk of war, according to Brussels-based think-tank International Crisis Group and others.
At the end of the day, "healthy and balanced development is the best form of conflict prevention", says Annan.
How to stop the shooting
There are two main approaches to bringing about a ceasefire: use military might to try and crush the enemy and force a surrender or withdrawal, or hold talks with the other side. In practice a ceasefire is often the result of both approaches.
The military solution alone usually fails to address the causes of the conflict – political, social or economic exclusion, or historical grievances – which makes long-term peace less likely. Even if armed groups are not the exclusive representatives of these concerns, they are often an expression of the unresolved issues, says international peace-building organisation Conciliation Resources.
There are strong arguments for adversaries to talk. These include the state's imperative to protect its civilians from mass atrocities, and the political fall-out if it fails in this duty. Armed groups too may lose support if they ignore this issue. There is also a pragmatic reason for governments to enter into talks: many armed groups use insurgency tactics to maintain the conflict against extraordinary odds. Such tactics often work even against a much stronger government army, so offering peace talks and concessions to rebels may be the only way to end the violence.
On the other hand, armed groups refuse to negotiate for a wide variety of reasons. They often vie for greater strength not only for military reasons but also to obtain greater clout in dictating the terms of peace. If an armed group is militarily weak, it may decide to bide its time or even adopt a more radical approach if it thinks this will strengthen its position in negotiations. The decision to negotiate can also be divisive and result in hardliners forming a splinter group that maintains the violent conflict.
Some may refuse to a ceasefire because they say it will give their enemies time to regroup, rearm and strengthen their position. Governments may also say that talks confer legitimacy to a group and its tactics.
Even when warring parties agree to a ceasefire, this does not automatically lead to official peace talks and it is not necessarily a sign of a genuine commitment to peace, especially if the parties have done so as a result of international pressure.
Local initiatives can play an important role in bringing about peace. In many parts of Africa, for example, community elders act as locally-respected independent mediators. And the importance of maintaining and strengthening family ties and community networks can also help resolve conflicts. Women with family ties to several different clans have a unique ability to build bridges between warring communities and pressure elders to mediate a ceasefire, research has shown.
Once the warring parties have agreed to sit around a table together, the way they engage in talks is crucial. The UNDP's Crisis Prevention and Recovery Bureau says participants must be willing to address the root causes of a crisis, not just the symptoms on the surface, to create genuine long-term peace.
Sometimes informal discussions are an important precursor to official peace talks. Peacemakers found in Northern Ireland and South Africa that war-torn societies shed their distrust, built relationships and bridged differences through informal dialogue. When civil war seemed imminent in Iraq, peace experts in 2007 and 2008 brought together Iraqis across the sectarian divide, together with former heads of the Irish Republican Army and African National Congress, to talk about conflict resolution. The purpose was not to hammer out an agreement overnight, but simply to get people talking. The talks helped ease violence in Iraq.
For more on peace talks, see this report by Conciliation Resources, "Choosing to engage: armed groups and peace processes" and UNDP's report "Why dialogue matters for conflict prevention and peacebuilding".
For more on the internationally-recognised principle that sovereign states, and the international community as a whole, have the responsibility to protect civilians from mass atrocities, click here.
The peace process does not end when the bullets stop flying. A ceasefire is just the start of a lengthy process of negotiating the terms of peace, addressing the root causes of the war, disarming and reintegrating ex-combatants, and addressing atrocities committed during the war.
A peace process usually takes years and needs careful handling to keep it on track and prevent war from restarting. International peacekeepers can play an important role in this.
The end of a civil war will often involve establishing an interim power-sharing government which includes former foes, in preparation for holding elections. Cross-border wars may result in lengthy talks on the demarcation of the shared border.
A crucial part of the process is the disarming, demobilisation and reintegration (DDR) of former combatants into society which helps create a secure environment for the peace process.
DDR programmes have been criticised for not taking into account the particular needs of girls and women in armed groups who often experience sexual violence and are stigmatised by their home communities when they return. There are DDR programmes specifically aimed at child soldiers but they lack funds and resources, says the Coalition to Stop the Use of Child Soldiers.
DDR programmes can be divisive. Communities who have suffered badly during conflict often say how unfair it can feel that they receive no official help to rebuild their lives, whereas those who killed and maimed their fellow citizens are given money or aid like farming tools.
It is not enough for rebel leaders and governments alone to negotiate peace terms – peace building must also happen in communities affected by war. They may be politically or economically marginalised, hold historic grievances and/or have experienced atrocities during the war and be seeking reparation.
Most agree that the more groups are involved in the peace process the better so their needs can be heard and addressed. This means not just all the warring parties, but also lawyers, peace activists, community leaders, women's groups, representatives of those displaced by the war, academics and international mediators.
The decision to deploy a peacekeeping mission rests with the U.N. Security Council, as part of its mandate to maintain international peace and security. The majority of missions are run by the United Nations, but the council can also authorise regional organisations like the North Atlantic Treaty Organization (NATO), European Union, African Union and coalitions of countries to carry out a mission.
The procedure begins with an initial flurry of consultations with the potential host government, regional organisations, U.N. agencies and countries that may be contributing peacekeepers and military equipment. The United Nations then deploys a team to assess the situation on the ground, and the secretary-general will normally write a report for the council based on its findings. If the council decides to deploy, it will pass a resolution outlining the size and mandate of the new mission.
Peacekeepers can be deployed to prevent a conflict developing or worsening, to avert a humanitarian disaster, or to help keep the peace. One or more of the parties to a conflict may insist that the United Nations play a role as a precondition for signing a peace agreement.
A U.N. peacekeeping mission can only succeed if the warring parties are genuinely committed to resolving the conflict through a political process. Without this commitment, the mission may become paralysed or be drawn into the conflict, say the U.N. guidelines for peacekeeping operations.
When the United Nations began peacekeeping in 1948, the "blue berets" as they became known focussed on maintaining ceasefires and stabilising situations on the ground. With the end of the Cold War and the change in the nature of war, missions turned to helping implement peace agreements.
Peacekeepers now include not just military personnel, but also people who can help build new state institutions – for example judiciary and police which incorporates former rebels – as well as human rights monitors, economists, electoral observers, de-miners, legal experts and humanitarian workers.
The United Nations has no army or police force, so it relies on member states to contribute staff and military equipment.
Peace negotiations often bring together people and groups who have committed atrocities and are eager to absolve themselves of responsibility. If they do not receive an amnesty from justice, there is the threat that they will restart the conflict and commit further crimes.
Yet justice issues, and the demands of the local and international communities, cannot be ignored, particularly because holding past perpetrators to account is vital to deterring future atrocities, says the International Crisis Group.
Cases can be tried by national or international courts. The International Criminal Court (ICC), based in The Hague, tries people accused of the most serious crimes – genocide, crimes against humanity and war crimes. It will only act if a case is not being investigated by a national court. The independent court was established in 2002, but its effective functioning is weakened by the United States' refusal to recognise the court's jurisdiction.
The peace for amnesty question has dogged ICC decisions. After it issued arrest warrants for atrocities committed by leaders of the rebel Lord's Resistance Army (LRA) in their war against the Ugandan government, the LRA vowed never to sign a final peace deal unless Kampala persuaded the ICC to drop the case.
Before the ICC was set up, international criminal tribunals were established to try crimes of war committed in former Yugoslavia and Rwanda. Joint national and international courts have been established more recently for serious crimes committed during conflicts in Cambodia, Sierra Leone, East Timor and Lebanon.
Some countries have established truth and reconciliation commissions to reveal past wrongdoing by a government or other important actors, in a bid to help resolve conflict. One notable example is South Africa's commission, established after apartheid, which invited both victims and perpetrators of violence and gross human rights violations to give their testimonies. The perpetrators who gave their testimonies could request amnesty from both civil and criminal prosecution.
But there is also a need for justice at community level in wars which were fought in towns and villages. This can take the form of local traditional justice systems, which seek to reintegrate former combatants into their home communities. This has been used by the Acholi people who bore the brunt of the LRA war in northern Uganda. Here combatants are often both the perpetrators and victims of crimes – many were abducted as children and then forced to commit atrocities in their home villages.
For more on international laws governing conflicts and international justice systems, see our briefing on international humanitarian law.
War-torn communities are not just battle-scarred and traumatised – they have lost breadwinners, crops, cattle, land, homes and businesses. Their villages and main roads may be littered with landmines which take years to remove and make it difficult to rebuild schools, hospitals and farm the land.
In Angola, roads, bridges and railways were either mined or destroyed during the country's lengthy civil war. The only way to reach many communities now is by air, which pushes up the cost of basic commodities and medicines – and makes it virtually impossible for people to sell their goods further afield.
Women who have been raped in war may be ostracised by their families and communities and forced to build a new life in towns and cities.
Families who fled during a conflict may return after many years to find their land or homes taken. Often the host communities have little or no infrastructure, schools, medical services or sanitation and are unable to cope with a large influx of people.
Some of the displaced may not want to go home, having spent all their lives in refugee or displacement camps where there are health clinics, schools and jobs.
There is a lot of good information about conflict resolution produced by research bodies and organisations actively involved in peace-building.
Here are some places to start your research: