The 15-member Security Council is by far the most powerful arm of the United Nations.
It can impose sanctions, as it did against Iran over its nuclear program, and authorize military intervention, as it did against Libya in 2011.
Its five permanent members are the victors of World War II: the United States, Britain, China, France and Russia.
The other 10 members are elected for two-year terms, with seats set aside for different regions of the world.
Efforts to expand the permanent membership of the council to include powers that have emerged since 1945 — such as India, Japan and Germany — have been stymied.
For every country that vies for a seat, rivals seek to block it.
Any member of the permanent five — or the P5, for short — can veto any measure, and each has regularly used this power to protect either itself or allies.
Since 1990, the United States has cast a veto on council resolutions 16 times, many concerning Israeli-Palestinian relations. Russia has done so 13 times, including four times over Syria.
The charter does allow the General Assembly to act if, because of a veto, international peace and security are threatened. But in reality, it is rarely done.
