For this sentence to be grammatically correct the verb ATTACK needs to be in accordance to the subject "an army of ants" which refers to the third person in singular "it". Therefore, the correct sentence would be as follows: In the 1998 movie Antz an army of ants attacks a rival group of termites.
It has to be considered the different kinds of subjects there are that refer to a group of subjects. Grammatically speaking these subjects need verbs in their singular form, for instance: A group of students was seen yesterday on the street.