Why does the international community intervene in some conflicts and not others, and what are the laws governing such interventions?
Here's a quick look at what's involved in setting up a peace mission.
Who makes the decision to form an international peace operation?
The decision to deploy a peace operation rests with the U.N. Security Council, as part of its mandate to maintain international peace and security.
The proposal for a new peace mission has to have a majority vote and no veto from the council's five permanent members - Russia, China, United States, France and Britain.
Most missions are run by the United Nations, but the Security Council can also authorise organisations like the North Atlantic Treaty Organization (NATO), European Union, African Union and coalitions of countries to carry out a mission.
The missions vary hugely. At one end of the spectrum they comprise a few monitors to report violations of a ceasefire agreement, at the other a fully armed military force.
The United Nations has no military or police force, so it relies on member states to contribute staff and military equipment.
Is the new brigade in Congo the first time U.N. peacekeepers have had the mandate to use force?
No, but it's rare for U.N. peacekeepers to have this mandate. It happened in Ivory Coast in 2004, and in Congo in the 1960s.
Most traditional peacekeeping missions fall under Chapter 6 of the U.N. charter which has measures to settle conflicts by peaceful means - including negotiation, mediation and confidence-building measures.
If Chapter 6 isn't enough, the Security Council can mandate the deployment of forces by land, sea or air to stop what's going on under Chapter 7, which is peace enforcement.
Several international peace operations without U.N. troops have had U.N. Chapter 7 mandates to use force, for example the NATO-led forces in Afghanistan.
What happens if the Security Council doesn't authorise a peace operation?
International law says there are only two cases when force is allowed: if a state is attacked, it can defend itself. And if there's a threat to international peace and security, Chapter 7 of the U.N. charter allows for international forces to be deployed.
If there's no U.N. mandate, countries may decide to go ahead anyway but their operations are considered illegal.
Most of the Security Council's permanent members have done this.
In 1999, the United States, France and Britain could not get Russia and China to agree on a coerced peace in Kosovo, so they went ahead anyway.
The United States and Britain failed to get a U.N. mandate to go into Iraq in 2003.
And Russia acted without a mandate when it sent troops into Georgia in 2008.
Does it matter if a peace operation is illegal?
Since 1945, there's been a lot of progress in developing a multilateral system to limit violence between states. Central to that system is the Security Council and the body of international law surrounding it.
But that system is undermined if its custodians - the permanent five on the Security Council - decide to go it alone.
How has peacekeeping changed since 1948?
When the United Nations began peacekeeping in 1948, the "blue berets" as they became known focussed on monitoring ceasefires and stabilising situations on the ground. They had no civilian protection mandates, no gender mandates, and no human rights mandates.
Their focus was mainly conflicts between states. The United Nations stayed out of domestic issues on the grounds of sovereignty and non-intervention.
With the end of the Cold War and the change in the nature of war, the United Nations began to think about what to do when states cannot or will not protect their own civilians.
In the 1990s countries began pressing for what was a new term, peace enforcement, which meant the use of force against groups carrying out acts against humanity.
The Security Council passed several resolutions invoking Chapter 7, including to provide protection for humanitarian aid in Somalia in 1992, and protection for civilians in Bosnia and Rwanda.
In 2005, the U.N. World Summit issued a statement saying that if states cannot or will not protect their civilians and there are massive and systematic violations of human rights or denial of humanitarian access, the United Nations can intervene under Chapters 6 and 7.
The first time the Security Council made official reference to the responsibility to protect civilians was in April 2006, in resolution 1674. The Security Council referred to that resolution when it authorised the deployment of U.N. peacekeeping troops to Darfur, Sudan, in August 2006.
Missions have also increasingly helped 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 - as well as human rights monitors, economists, electoral observers, de-miners, legal experts and humanitarian workers.