Throughout this period, racial tensions exploded, most violently in Chicago, and lynchings—mob-directed hangings, usually racially motivated—increased dramatically in the 1920s. Urban riots—whites attacking blacks—became a northern and western problem. Many whites defended their space with violence, intimidation, or legal tactics toward African Americans, while many other whites migrated to more racially homogeneous suburban or exurban regions, a process known as white flight. Racially restrictive housing covenants were ruled unenforceable under the Fourteenth Amendment in the 1948 landmark Supreme Court case "Shelley v. Kraemer".