"National security" can be a loosely defined term subject to abuse by elected officials. It can be used to justify acts that violate federal or state laws, or city or county ordinances. During the late 1960s and early 1970s, federal agencies including the Federal Bureau of Investigation and the Central Intelligence Agency exceeded their mandates and legal authorities in monitoring, and sometimes acting against, protest movements, mostly involving the Vietnam War. The C.I.A.'s role in such domestic activites became the basis of the Church Committee hearings in 1975-76, which also revealed to the public C.I.A. activities abroad that ran counter to U.S. democratic values, if not always U.S. or international law.
The development that led to the revelations was the Watergate scandal, in which individuals, working at the direction of the Nixon Administration, carried out illegal activities, including the break-in of Daniel Ellsberg's psychiatrist's office (Ellsberg was a former government national security analyst who leaked classified documents to the New York Times that became known as "the Pentagon Papers.") and, of course, the break-in and wiretapping of Democratic National Committee offices in the Watergate building in Washington, D.C.
More recently, there are concerns about the increasingly pervasive role of government surveillance of U.S. cities in the name of national security. These developments, spurred by the terrorist attacks of September 11, 2001, could, according to some analysts and civil libertarians, pose a threat to the constitutional right to privacy enjoyed by the American people. Some actions taken to protect against terrorist attack could come into conflict with state and local laws and ordinances.
Law enforcement agencies have historically been very "turf conscious," meaning they jealously guard their areas of jurisdiction and resist sharing information with other agencies. One of the outcomes of the 9/11 terrorist attacks was a major effort by Congress to remove legal and structural barriers to inter-agency cooperation at all levels of government. Toward this end, the national security imperative of the federal government has become part of the core mission of local law enforcement.
The F.B.I. has, since its inception, rankled local police forces by taking over investigations that crossed into federal jurisdiction and refusing to share information and credit for successes. The post-9/11 changes have resulted in closer relationships and greatly increased sharing of information among agencies. The "national security" argument can still result in federal activities that conflict with local law enforcement, but there is much more cooperation today than before 9/11.