Ally (verb)

1. To become united with another country or group, especially for a political or military purpose.
2. To join with others in a common cause.

Origin:

Late middle english: from old french alier, from latin alligare "bind, tie to", from ad- "to" + ligare "bind".

Examples:

1. The two countries allied against a common enemy.
2. He allied with other businessmen to form a company.
3. The company allied with a competitor to increase market share.
4. The two political parties allied to form a coalition government.
5. The two countries allied to promote peace and stability.