Collective defense is an arrangement, usually formalized by a treaty and an organization, among participant states that commit support in defense of a member state if it is attacked by another state outside the organization.

NATO is the best known collective defense organization. Its now famous Article V calls on (but not fully commits) member states to assist another member under attack. This article was invoked after the September 11 attacks on the United States, after which other NATO members provided assistance to the US War on Terror in Afghanistan. NATO 2002 Summit in Prague The North Atlantic Treaty Organisation[1] (NATO), also called the North Atlantic Alliance, the Atlantic Alliance or the Western Alliance, is an international organisation for collective security established in 1949, in support of the North Atlantic Treaty signed in Washington, DC, on 4 April 1949. ... A sequential look at United Flight 175 crashing into the south tower of the World Trade Center The September 11, 2001 attacks (often referred to as 9/11—pronounced nine eleven or nine one one) consisted of a series of coordinated terrorist[1] suicide attacks upon the United States, predominantly... The war on terrorism or war on terror (abbreviated in U.S. policy circles as GWOT for Global War on Terror) is an effort by the governments of the United States and its principal allies to destroy groups deemed to be terrorist (primarily radical Islamist organizations such as al-Qaeda...

Collective defense has its roots in multiparty alliances, and entails benefits as well as risks. On the one hand, by combining and pooling resources, it can reduce any single state's cost of providing fully for its security. Smaller members of NATO, for example, have leeway to invest a greater proportion of their budget on non-military priorities, such as education or health, since they can count on other members to come to their defense, if needed. Alliances can refer to: Alliances, an expansion for the trading card game Magic: The Gathering The plural of alliance This is a disambiguation page — a list of articles associated with the same title. ...

On the other hand, collective defense also involves risky commitments. Member states can become embroiled in costly wars from which it was neither the direct victim nor the aggressor benefit. In the First World War, countries in the collective defense arrangement known as the Triple Entente (France, Britain, Russia) got pulled into war quickly when Germany declared war on Russia. Ypres, 1917, in the vicinity of the Battle of Passchendaele. ... European military alliances in 1915. ...

Although they overlap, collective defense is different from collective security, in which members work to deter aggression in the first place. The United Nations is an example of a collective security arrangement, albeit with significant limitations. In general, collective defense involves a more explicit commitment among a group of like-minded members. Collective Security is a system aspiring to the maintenance of peace, in which participants agree that any breach of the peace is to be declared to be of concern to all the participating states, and will result in a collective response. ... The United Nations (UN) is an international organization whose stated aims are to facilitate co-operation in international law, international security, economic development, and social equity. ...

Another examples of collective defense is West African organisation ECOMOG and Collective Security Treaty Organization (CSTO).  Western Africa (UN subregion)  Maghreb West Africa or Western Africa is the westernmost region of the African continent. ... ECOMOG is a West African multilateral armed force established by the Economic Community of West African States (ECOWAS). ... The Commonwealth of Independent States (CIS) (in Russian: Содружество Независимых Государств (СНГ) - Sodruzhestvo Nezavisimykh Gosudarstv) is a confederation or alliance consisting of 12 former Soviet Republics: Armenia, Azerbaijan, Belarus, Georgia, Kazakhstan, Kyrgyzstan, Moldova, Russia, Tajikistan, Turkmenistan, Ukraine, and Uzbekistan. ...

See also

• List of Military alliances
• Collective security