However, static emplacements remain useful in both defense against naval attacks and defense against air attacks. Naval mines are still an inexpensive but effective way to defend ports and choke off supply lines. Large static air defense systems that combine antiaircraft guns with missile launchers are still the best way to defend against air attacks. Such systems were used effectively by the North Vietnamese around Hanoi. Also, the United States has invested considerable time and money into the construction of a National Missile Defense system, a static defense grid intended to intercept nuclear intercontinental ballistic missiles.