aliar

To ally, join forces, or form an alliance — most often used reflexively, aliarse con/contra alguien, in political, military, and business contexts alike: los países se aliaron contra el enemigo común. Outside formal contexts it also works for simple cooperation, like two coworkers teaming up on a project.

• Los dos países se aliaron para enfrentar la crisis. — The two countries allied to face the crisis.
• Se alió con su vecino para denunciar el ruido. — She teamed up with her neighbor to report the noise.
• Las empresas rivales decidieron aliarse en este proyecto. — The rival companies decided to join forces on this project.

Aliar is almost always used reflexively (aliarse) when people or countries form an alliance with each other — dropping the se and saying just alié con reads as incomplete to a native speaker.