An alliance is an agreement between two or more parties. It is made to benefit both parties.
The word "alliance" is used mainly in war. When two or more countries make an alliance, that means they promise to help each other during war. Countries that have an alliance are called allies. When talking about countries that have an alliance, people say they are allied.
There can be only two countries in an alliance, or there can be more than two countries in an alliance. An example of an alliance with more than two countries is the Central Powers in World War I. This alliance was made up of Germany, Austria-Hungary, The Ottoman Empire, and Bulgaria.
Allies Day, May 1917, National Gallery of Art
Alliance for Kids. Kiddle Encyclopedia.