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 the South Vietnamese during the Vietnam War.
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".
The Allies 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.
Scholars are divided as to the impact of alliances. Several studies find that defensive alliances deter conflict. One study questions these findings, showing that alliance commitments deterred conflict in the prenuclear era but has no statistically meaningful impact on war in the postnuclear era. Another study finds that while alliance commitments deter conflict between sides with a recent history of conflict, alliances tend to provoke conflicts between states without such a history.
A 2003 study found that allies fulfill their alliance commitments approximately 75% of the time.
According to a 2017 poll by WIN/GIA, the United States was the most preferred ally internationally. Russia and China, who preferred one another, both trailed the US globally. Four countries, Bulgaria, Greece, Slovenia and Turkey, preferred Russia, despite being members of NATO.
In Pakistan, 72% of respondents preferred ties to China, the largest margin of any country surveyed, while 46% of Bangladesh preferred India. A total of 22 countries indicated a preference for the United Kingdom at a rate of 10% or more, but the United States was the only country to prefer the UK over any other, at a rate of 43%. Five counties preferred France at a rate of 10% or more, led by Belgum at a rate of 25%. A single country, Iraq expressed no preference, while three other countries, Lebanon, Palestine, and Slovenia, expressed no preference at a rate of 10% or more, although at a smaller rate than their preference for Russia on the part of Lebanon and Slovenia, and China on the part of Palestine. Kosovo reported the most unified opinion, preferring the US at a rate of 92%, while Russia's most unified supporters were Mongolia (71%), Armenia (67%) and Serbia (56%). In total, 21 countries expressed a preference for the US at a rate of 50% or more.