A State of Emergency is declared at times of overwhelming danger. During such times, certain normal standards of procedure are abrogated and replaced by others. In some situations, martial law is declared, allowing the military greater authority to act.
In the United States, the chief executive is typically empowered to declare a State of Emergency. The President of the United States, a governor of a state, or even a local mayor may declare a State of Emergency within his or her jurisdiction. This is relatively rare at the federal level, but quite common at the state level in response to natural disasters.
Typically, a state of emergency empowers the executive to name coordinating officials to deal with the emergency and to override normal administrative processes regarding the passage of administrative rules.
Depending on the needs of that emergency, people may be arrested without cause, private places may be searched without warrant, or private property may be seized without immediate compensation or a chance to prior appeal.
The courts in the United States are often very lenient in allowing almost any action to be taken in the case of such a declared emergency, if it is reasonably related. For example, habeas corpus is the right to challenge an arrest in court. The U.S. Constitution says, "The Privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it."
Habeas corpus was suspended on April 27, 1861 during the American Civil War by Abraham Lincoln in parts of midwestern states, including southern Indiana. He did so in response to demands by generals to set up military courts to rein in "Copperheads", or those in the Union who supported the Confederate cause. Lambdin Milligan and four others were accused of planning to steal Union weapons and invade Union prisoner-of-war camps and were sentenced to hang by a military court in 1864. However, their execution was not set until May 1865, so they were able to argue the case after the Civil War. It was decided in the Supreme Court case Ex Parte Milligan 71 US 2 1866 that the suspension was unconstitutional because civilian courts were still operating, and the Constitution (according to the Court) only provided for suspension of habeas corpus if these courts are actually forced closed.
In United States history, there have been multiple federal states of emergency declared, some of which have never technically been ended.