The war on illegal immigration was fought at the border Saturday, but it was not the Mexican border. It was Bergen County's. On the street that divides Dumont from Bergenfield, the aspirations of illegal immigrants seeking the American Dream and the frustrations of many Americans who seek to stop illegal immigration were vividly on display. On the Bergenfield side of Columbus Avenue, about 30 immigrant day laborers waited for employers to pick them up. On the Dumont side, about 20 North Jersey residents held American flags and signs noting: "It's illegal to hire illegal aliens."