The Soviet Union evaporated in the early 1990s, but many of it's nuclear missiles did not. More ominously, everyone noted that Iran, Iraq and North Korea were working on long range missiles and nuclear weapons. This is where the Doomsday Math comes in.
The justification for keeping the missile defense efforts going through the 1990s and into the 21st century is the possibility that eventually Iran, Iraq, North Korea or some other hostile nation will get their hands on nuclear weapons and long range missiles to carry them to North America. At the end of the Cold War there were some 40,000 nuclear weapons, about half that number was deliverable by aircraft or long range missile. Some 10,000 of these warheads were aimed at the United States. Today there about 15,000 nuclear weapons on the planet. Some 7,000 belong to the United States, and these are in pretty good shape. Russia still has some 6,000 nukes, but far fewer of them are in any condition to be used. This is where we must consider a dirty little secret of the cold war. Many of the nuclear missiles were not expected to actually work under wartime conditions. The reasons are many, many having to do with the design of the electronics and quality of the maintenance. One reason the Soviet Union had 10,000 warheads aimed at us was that even if most of the missiles failed, there would still be enough to vaporize us.
Ten years later, Russia can still get about a thousand warheads on target, out of the 6,000 they still posses. China has a few hundred warheads and fewer than ten missiles capable of reaching western North America. No other potential foe has missiles that can reach the United States. But if you are planning a missile defense system, you have to plan for what will be out there ten years in the future. This is where the debate gets interesting.
A missile defense system that can stop a few incoming missiles will cost over a hundred billion dollars. Probably closer to two hundred billion. This means spending a fraction of one percent of annual GNP on this project. Nations like Iran, Iraq and North Korea can develop a nuclear equipped ICBM is they are willing to spend enough money. A few percent of GNP over five or ten years will do it. What are the chances of that happening. Well, let's consider other things that are going on in these nations. Iran has a growing reform movement. Iranians are tiered of all the fundamentalist rhetoric, religious rule and want some prosperity. It's unlikely that Iran will reverse it's current trend and put a lot of money into building nukes and ICBMs. Possible, but not very likely. Iraq has the money and a police state to keep people in line while money is spent to produce ICBMs. But Iraq first has to get rid of UN blockade. Then Iraq has to worry about Israel. Middle East politics cannot be ignored, and Israel is Iraq's principal foe. If Iraq gets close to having a nuclear missile, Israel will attack. Israel did this in 1982, destroying a reactor that was a key component of Iraq's nuclear weapons program. The chances of Iraq keeping it all together until it has an ICBM that can reach American are slim indeed. And then there is North Korea. The country is broke, and the situation is getting worse by the day. In ten years it's more likely that North Korea will have been absorbed by South Korea. While it's true that the north has starved over a million of it's citizens to death to support it's massive army and ICBM program, it's also true that such heroic efforts have failed in the past. The prospects of an individual like Osama Bin Laden building an ICBM only happens in movies.
Beyond the practical problems of mobilizing resources to build nukes and an ICBM, there is also the retaliation factor. Launch a nuke at the United States and our satellites will immediately identify the launch site. If a lot of Americans are killed, the "Remember Pearl Harbor" reflex will take over and the launching nation will disappear in a radioactive cloud.
Anti-missile defenses can be made to work. America has a track record of making such heroic projects work. But in this case, why?
Doomsday Math; For over four decades, hundreds of billions of dollars have been spent on developing weapons to stop nuclear missiles. In the past, these efforts faltered when it was realized that such a system would make the enemy more trigger happy. This was during the Cold War, and it took the United States a while to realize that to the paranoid Soviet Union, American missile defenses were an offensive weapon. By Soviet logic, we were building missile defenses so we could nuke Russia without fear of retaliation. In the 1970s, both sides agreed to refrain from working on missile defenses. This calmed down the Russians, who had been making noises about "launching a first strike" before the American missile defenses went into service. But in the 1980s, the United States began work on an even more ambitious missile defense system; "Star Wars." This turned out to be a clever move. Doing research and development on missile defense was not against the treaty, but actually putting the system into operation was. The Cold War ended before the U.S. got it's "Star Wars" system working, so we never found out how the Russians would have reacted. The ambitious, and expensive, new technology proposed for Star Wars played a part in the collapse of the Soviet Union. The Russians later admitted that the prospect of trying to match the Star Wars effort demoralized the Soviet leadership.