In everyday English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 usage, allies are people, groups, or nations that have joined together in an association for mutual benefit or to achieve some common purpose, whether or not explicit agreement has been worked out between them. When the term is used in the context of war

War

War is a state of organized, armed, and often prolonged conflict carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political...

 or armed struggle, such associations may also be called Allied Powers, especially when discussing World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

 or World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

.

A formal military alliance is not required for being perceived as an ally—co-belligerence

Co-belligerence

Co-belligerence is the waging of a war in cooperation against a common enemy without a formal treaty of military alliance.Co-belligerence is a broader and less precise status of wartime partnership than a formal military alliance. Co-belligerents may support each other materially, exchange...

, fighting alongside someone, is enough.