1. To join or combine with someone or something in order to oppose someone or something else. Though normally distrustful of each other, the two countries decided to unite against the growing threat from their neighbor to the north. Though we may have different beliefs and opinions, we must remain united against hate and oppression.

2. To cause multiple people, groups, or organizations to join or combine together in opposition to someone or something else. In this usage, a noun or pronoun is used between "unite" and "against." The unprovoked attack united the entire region against the small militant nation. The execution of the rebels united the citizens of the country against the colonial rule of the foreign kingdom.