Probably god was not on the side of the North Vietnamese, as they were communists, that don't believe in a god.
I was talking recently with a christian friend who is very much against all war and killing. He also found the "god is on our side" a queer statement. His view is that god said in the ten commandments, "thou shalt not kill", and there wasn't a list of conditions after this commandment, saying "but in these situations its ok to kill".
As a non-christian, I share his view about not killing, I personally do not think that there is any valid reason for killing another human being. I also hope that I personally never have to be tested on my conviction, and will do everything I can to prevent this.