More fighting in the Mexican Crisis today when American troops participated in a Joint raid with Mexican Nationals against rebel racket positions in Cancun.
There's no more about that from drafts of the scripts or the movie transcript. Like many anticipation/SF movies, they set some background with big or small discrepancies between the two eras to back up the idea of it being in the future.
What these rebels are fighting isn't said. Why would the USA be helping the mexican government? Because it's common interest, as it's often the case when countries are involved, especially neighbours. This is not said either in the movie.