Forty years ago, President Ronald Reagan unveiled the Strategic Defense Initiative (SDI). As long as nuclear weapons exist, the United States would maintain credible deterrence against its nuclear rivals, and President Reagan made sure of that. He believed it was unwise and wrong to leave Americans vulnerable to nuclear attack if deterrence failed. He understood that active defenses were a necessary component to the suite of options for the United States to protect Americans. The aim to bolster U.S. homeland defense is as important today as it was in 1983.