Collective security is arrangement between states in which the institution accepts that an attack on one state is the concern of all and merits a collective response to threats by all. Collective security was a key principle underpinning the League of Nations and the United Nations. Collective security is more ambitious than systems of alliance security or collective defense in that it seeks to encompass the totality of states within a region or indeed globally.

The premise of a collective security arrangement is that it serves as a deterrent to aggression by committing an international coalition against any aggressor. While collective security is an idea with a long history, its implementation in practice has proved problematic.

Collective security is also referred to by the phrase "an attack on one is an attack on all". However, usage of this phrase also frequently refers to Article 5 of the North Atlantic Treaty, the collective security provision in NATO's charter.

Collective Security Policy is referred to the diplomatic attempts of USSR to achieve a collective security alliance with France, Poland, Czechoslovakia and Baltic countries against Nazi Germany in interwar period.

It is an important foreign policy put forward by the Soviet Union in the 1930s to stop the aggression of fascist countries and maintain world peace and national security.

Collective security is one of the most promising approaches for peace and a valuable device for power management on an international scale. Cardinal Richelieu proposed a scheme for collective security in 1629, which was partially reflected in the 1648 Peace of Westphalia. In the eighteenth century many proposals were made for collective security arrangements, especially in Europe.

The concept of a peaceful community of nations was outlined in 1795 in Immanuel Kant's Perpetual Peace: A Philosophical Sketch. Kant outlined the idea of a league of nations that would control conflict and promote peace between states. However, he argues for the establishment of a peaceful world community not in a sense that there be a global government but in the hope that each state would declare itself as a free state that respects its citizens and welcomes foreign visitors as fellow rational beings. His key argument is that a union of free states would promote peaceful society worldwide: therefore, in his view, there can be a perpetual peace shaped by the international community rather than by a world government.

International cooperation to promote collective security originated in the Concert of Europe that developed after the Napoleonic Wars in the nineteenth century in an attempt to maintain the status quo between European states and so avoid war. This period also saw the development of international law with the first Geneva Conventions establishing laws about humanitarian relief during war and the international Hague Conventions of 1899 and 1907 governing rules of war and the peaceful settlement of international disputes.