I thought it was Hell Hounds by Ernest Bywater, which has an invasion from Mexico, but I am looking for a similar story which contains the following -
Invasion from Mexico, National Guard units do much of the fighting, at one point they are warned of Friendlies from another state joining the fight.
Sadly that is the only scene I can remember :(
Ernest, I have just finished Hell Hounds again - Thank You, Regards G_R