Vietnam War and Cold War spy scandals caused Congress to reassert its power over foreign affairs in 1970s
War Powers Act of 1973 limited presidents' power to deploy troops without receiving an official act of war from Congress
In recent years, War on Terror has tested limits of executive power in times of international crisis
Eventually, Congress would decide that this trend had gone too far. A series of revelations about CIA covert operations during the late 1960s and early 1970s (assassination attempts against Cuba's head of state and secret assistance to anti-government plotters in Chile, to name a couple), led Congress to pass new legislation requiring that the CIA make periodic reports to a Congressional intelligence committee. These efforts to restrict the CIA were part of a larger attempt to rein in the presidency. And it was not just the president's foreign policy powers that raised concerns. President Richard Nixon's impoundment of federal monies (that is, his refusal to spend Congressional appropriations with which he disagreed), and his claim that he did not need to answer a court order because he possessed "executive privilege," led many to argue that the president's domestic behavior was also out of control. But it was in the area of foreign policy that these critics of executive power voiced their greatest concerns—in particular, Nixon's continuation of the war in Vietnam without a formal declaration of war from Congress led some to bewail the rise of the "imperial president."
Since the early 1960s, Presidents Johnson and Nixon had waged war in Vietnam without securing a formal congressional declaration of war. Both presidents claimed that the imprecisely worded Tonkin Gulf Resolution of 1964 provided them with the congressional authorization that they needed. But Congress disagreed—and so in 1973, it passed the War Powers Act in order to prevent future presidents from engaging in undeclared military conflicts. Under the act's terms, the president is required to notify Congress within 48 hours of deploying troops "into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances."