The United Nations came into being in 1945, when 51 member nations agreed to form an organization to promote world peace and justice and to push for better living standards for people around the world. In the years following its formation, it became a forum for mediating Cold War disputes between the United States and the Soviet Union. It also could play a military function as well, as UN intervention in Korea demonstrated. The UN also encouraged the decolonization of African and Asian nations, and attempted to manage the conflicts that emerged in some of these countries.
Over the years, the most visible role of the UN has been in the area of "peacekeeping," which is actually not called for in its original charter but has always been a major priority. UN forces have been deployed as observers and peacekeepers in war-torn regions from Palestine to Rwanda. In recent years, these interventions have mostly centered on civil conflicts, and have aroused much controversy. Some have argued that the UN has been too heavy-handed in these interventions, others that the troops deployed by the organization lack the firepower and the legal authority to make a serious difference. Many also question the UN's support for invasions of nations by other nations, particularly the United States.
As the recent crisis in Syria has demonstrated, another related problem facing the UN when it comes to foreign intervention is the fact that one nation (Russia) on the Security Council can veto any intervention. Today, the United Nations has expanded its membership to almost 200 nations, and its role have expanded to include world law enforcement, negotiating loans to developing nations, managing global environmental issues, and combatting terrorism.