Liberals scoffed in 1983 when President Ronald Reagan first proposed the Strategic Defense Initiative missile defense system. They called it "Star Wars" and claimed the technology was fictional. Other critics took a different tack, arguing that Star Wars would be harmful if it did work because it would upset the delicate nuclear balance between the United States and Soviet Union.

The Soviets depended on the threat of mutual assured destruction in order to maintain diplomatic parity with a wealthier, more advanced adversary. To disturb that balance, some analysts argued, would destabilize the international order. Of course, the Soviets, whose entire system of government was rooted in the use of terror, opposed even a shared American-Soviet missile defense system for precisely this reason.

Three decades after Reagan's landmark proposal, the technical issue is moot. This week, the world has watched Israel's Iron Dome system -- partially funded by the U.S. and incorporating U.S. technology -- perform remarkably well against more than 1,000 Hamas rockets fired from the Gaza Strip. When rockets are launched, Iron Dome calculates almost instantly whether they are headed for populated areas, then intercepts them as needed. The Israeli military says Iron Dome has been 87 to 90 percent effective and is the key reason why there's been only one Israeli civilian fatality.

Iron Dome has also minimized the prospect of another bloody ground war like the one Israel waged in Gaza in December 2008 and January 2009 in order to stop similar rocket attacks. That clash resulted in more than 1,400 Palestinian deaths and 5,000 wounded, with minimal Israeli losses. So Iron Dome has also saved countless Palestinian civilians.

Even so, as in the 1980s, some critics complain now that Iron Dome has upset the balance in Israeli-Palestinian relations. Yoav Fromer lamented in the Washington Post earlier this week that it has taken pressure off Israel to “wage diplomacy to end violence that mandated Iron Dome in the first place.”

Regardless of what one thinks of the long-term conflict, Israel's response in Gaza or the Israeli willingness to make concessions, Iron Dome has deprived Hamas of missile terrorism against random civilians as diplomatic leverage -- just as SDI threatened to stop the Soviets from using international nuclear terror for diplomatic leverage against America. How is this bad? If there can be no negotiation without constant threats of random, lethal attacks on civilians, the conditions for fruitful negotiation likely don't exist.

For the same reason, President Obama erred in 2013 with his decision to scrap -- in response to Russian objections -- a planned system in Europe to defend against missile threats from the Middle East. Obama won no good will from the Russians, even as he forfeited an important opportunity to strengthen U.S. allies against future threats. Missile defense allows strong nations to defend themselves against weaker enemies without resorting to terrorist tactics or fighting bloody wars. This is a win-win, except for terrorists like Hamas and national rulers bent on aggression against neighbors.