An alliance is a relationship among people, groups, or states that have joined together for mutual benefit or to achieve some common purpose, whether or not explicit agreement has been worked out among them. Members of an alliance are called allies. Alliances form in many settings, including political alliances, military alliances, and business alliances. When the term is used in the context of war or armed struggle, such associations may also be called allied powers, especially when discussing World War I or World War II.

A formal military alliance is not required for being perceived as an ally—co-belligerence, fighting alongside someone, is enough. According to this usage, allies become so not when concluding an alliance treaty but when struck by war.

When spelled with a capital "A", "Allies" usually denotes the countries who fought together against the Central Powers in World War I (the Allies of World War I), or those who fought against the Axis Powers in World War II (the Allies of World War II). The term has also been used by the United States Army to describe the countries that gave assistance to South Vietnam during the Vietnam War.

The Allied Powers in World War I (also known as the Entente Powers) were initially the United Kingdom, France, the Russian Empire, Belgium, Serbia, Montenegro and Japan, joined later by Italy, Portugal, Romania, the United States, Greece and Brazil. Some, such as the Russian Empire, withdrew from the war before the armistice due to revolution or defeat.

After the end of World War II and during the Cold War, the North Atlantic Treaty Organization (NATO) was formed as a political and military alliance that promotes anti-communist values.

More recently, the term "Allied forces" has also been used to describe the coalition of the Gulf War, as opposed to forces the Multi-National Forces in Iraq which are commonly referred to as "Coalition forces" or, as by the George W. Bush administration, "the coalition of the willing".