1.to join, or enlist somebody, in an association with one or more other states, organizations, or people for a common purpose
2.to connect something with something else through similarity or common features
3.to connect people or families, or form a connection with another person or family, especially through marriage
4.if a country allies itself with another country, it makes an agreement with it that they will work together to help each other, especially in a war
5.if you ally yourself with someone, you work together to help each other
6.if you ally yourself with something such as a belief or an organization, you show your strong support for it