THE RACES: This Is a Battlefield

Marquette Park in southwestern Chicago is one of the city's largest green spaces—a 321-acre expanse of grassy meadows, tennis courts, fishing lagoons and a golf course. Surrounding the park is a white "ethnic" community of 11,000 Lithuanian, Irish and Polish families—a vigorous old neighborhood that has tenaciously barred blacks and preserved itself as one of the city's last desirable white areas.

Two weekends ago a ragtag line of some 150 blacks and whites, members of an obscure group called the Martin Luther King Jr. Movement, decided to challenge the discriminatory patterns in the Marquette Park area. Their tiny demonstration brought...