The Security Council was constructed as an organ with primary responsibility for preserving peace. Unlike the General Assembly, it was given power to enforce measures and was organized as a compact executive organ. Also unlike the assembly, the Security Council in theory functions continuously at the seat of the United Nations.
The council has 15 members. Five—China, France, Great Britain, the United States, and Russia (until 1991 the USSR)—are permanent. The 10 nonpermanent members are elected for two-year terms by the General Assembly; equitable geographic distribution is required. Customarily there are five nonpermanent members from African and Asian states, one from Eastern Europe, two from Latin America, and two from Western Europe and elsewhere. In the council the presidency is occupied for one-month terms in the alphabetical order of the members' names in English.
There are two systems of voting in the Security Council. On procedural matters the affirmative vote of any nine members is necessary, but on substantive matters the nine affirmative votes required must include those of the five permanent members. This requirement of Big Five unanimity embodies the so-called veto. The veto has prevented much substantive action by the UN, but it embodies the reality that resolution of major crises requires agreement of the major powers.
Under the Charter the council may take measures on any danger to world peace. It may act upon complaint of a member or of a nonmember, on notification by the secretary general or by the General Assembly, or of its own volition. In general the council considers matters of two sorts. "Disputes" (or situations that may give rise to them) that might endanger peace. Here the council is limited to making recommendations to the parties after it has exhausted other methods of reaching a solution. In the case of more serious matters, such as "threats to the peace," "breaches of the peace," and "acts of aggression," the council may take enforcement measures. These may range from full or partial rupture of economic or diplomatic relations to military operations of any scope deemed necessary. By the terms of the Charter, the United Nations was forbidden to intervene in matters "which are essentially . . . domestic," but this limitation was not intended to hinder Security Council measures to prevent threats to peace. The UN Charter was intentionally ambiguous regarding domestic issues that could also be construed as threats to peace, and left a potential opening for intervention in domestic issues that threatened dangerous international repercussions.