Resource Library




War is generally defined as violent conflict between states or nations.


5 - 8


Civics, Social Studies


Tank in Iraq Invasion

A United States Army 3rd Infantry Division M1/A1 Abrahms tank during the 2003 invasion of Iraq. While the use of force was authorized by Congress, like many U.S. military conflicts, war was not declared.

Photograph by Scott Nelson/Getty Images

War is generally defined as violent conflict between states or nations.

Nations go to war for a variety of reasons. It has been argued that a nation will go to war if the benefits of war are deemed to outweigh the disadvantages, and if there is a sense that there is not another mutually agreeable solution. More specifically, some have argued that wars are fought primarily for economic, religious, and political reasons. Others have claimed that most wars today are fought for ideological reasons.

In the United States, the legal power to declare war is vested in Congress; however, the president is the commander-in-chief of the military, so he or she holds power to conduct a war once it has been declared. In many instances, the president has used military force without declaring war.

Just War Theory

In Western tradition, there is a sense that the reasons for war must be just. This idea dates back to ancient times, but is most clearly traced to the writings of St. Augustine and St. Thomas Aquinas. They attempted to justify war, and reconcile it with the Christian belief that taking a human life is wrong.

To Aquinas, a war must be just in both the reasons for going to war and how war is fought. Reasons for going to war—jus ad bellum—are just if (1) war is declared by an appropriate authority; (2) the war is waged for a just cause; and (3) the war is waged for just intentions. An appropriate authority is a proper, governing authority. A “just cause” may include self-defense or a response to injustice. “Just intentions” mean that it must not be fought for self-interest, but for justice or a common good. In addition, (4) there must be a reasonable chance of success; (5) the good that will be achieved must outweigh the bad; and (6) war must be a last resort.

Once just reasons for going to war are satisfied, conduct in the war—jus in bello—must be just as well. Just conduct in a war means that it must be specific and proportional. That is, noncombatants and civilians must not be deliberately targeted. Further, only such force as is necessary must be used, and harms must be proportionate to the goal sought.

Law of War

Some of the just war theories have been adopted as parts of international agreements and incorporated into the law of war (i.e., international law) that regulates the resort to armed force, the conduct of hostilities, and the protection of war victims.

The Geneva Conventions, for example, are a series of international treaties that are designed to protect noncombatants, civilians, and prisoners of war. The treaties were negotiated in Geneva, Switzerland, between 1864 and 1977. The First and Second Geneva Conventions apply to sick and wounded soldiers and sailors. They contain provisions related to protecting the wounded and sick, as well as medical personnel and transports. The Third Geneva Convention applies to prisoners of war, and the Fourth Geneva Convention applies to people in occupied territories. The Third Convention requires humane treatment of prisoners, including adequate food and water. The Fourth Convention includes provisions that forbid torture and the taking of hostages, as well as provisions related to medical care and hospitals.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Tyson Brown, National Geographic Society
National Geographic Society
Production Managers
Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society
Program Specialists
Sarah Appleton, National Geographic Society, National Geographic Society
Margot Willis, National Geographic Society
Clint Parks
Last Updated

May 20, 2022

For information on user permissions, please read our Terms of Service. If you have questions about licensing content on this page, please contact for more information and to obtain a license. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. She or he will best know the preferred format. When you reach out to him or her, you will need the page title, URL, and the date you accessed the resource.


If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.


Text on this page is printable and can be used according to our Terms of Service.


Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.

Related Resources