Assume for the purpose of this argument that missile defense is viable and within the current reach of technology.
What the heck took so long for them to decide on this, I mean what are the bad points of have a massive missile defense capability and a massive second-strike capability.
The two combined make it where you just don't want to screw with us, and if you do screw with us, we will screw you up more.
In the ole cold war days of yore a massive second-strike capability assured destruction for both. This operated on the principle that the major players involved did not want to be destroyed in the first place. Given countries with leader like Iran's supreme leader Ali Khamenei who is the apparent mouthpiece of God, and the crazy North Korean leader. The premise that countries want attack because they don't want their people to be destroyed comes into serious question.
Once you have an outstanding missile defense system (An ideal one would be capable of sensing and responding quickly to threats all over the world. That is why we need Japan's help to position radar stations in a lot of random places.), you would have an excellent offense and a great, but widely untested defense system.
The enemy would have a few options if they were to attack America.
They could build up a truly massive missiles system that once used would allow a few missiles to sneak by.
This has a few problems.
We would know what the enemy was trying to do (the amount of missiles it would take would be tremendous, if we did the system correctly). This method would be very expensive, because you would have to assume that any of the missiles fired have the ability to get though. All of the fired missiles would need to be equal to promise maximum destructive power, unless you were able to find a hole in the defensive net through intelligence or strategic forethought.
Also something to consider with the above point would be the quality of missiles. One thing that would hurt there chances of getting missiles though is that we are several years ahead of most countries, as far as missiles technology is concerned. This would hurt their chances of getting any though.
Choosing such a path would seriously hurt the nation in question in the realm of international relations. Well, it would hurt that nation with other nations who did not have a defensive net of protection. We would not care. We would only point it out to the world as an example of what we are protecting our selves from.
The country could attack with a few missiles that have defensive measures (this just will make us develop defensive measures against defensive measures, which very few countries could keep pace with the US in the area of technological innovation).
Or the county could attack with one bomb smuggled in onto American soil. Our fabulous defensive net of protection will not help us out there.
Now there is the issue of terrorist organizations, they would most likely attack using the last method mentioned. So we still need to investigate further measures for protecting American soil. But the missile defense is a good broad start, and would limit the methods of attacking American soil to actually being on American soil. Which I think America should use their agents in the fields (the little old ladies and gun toting militants are a good start, then we can work on recruiting the every day American to just call 911 if they see a big bomb) for surveillance activates that would be necessary to catch such attacks.
Also, a defensive net would allow us to keep our status as top dog of the world. The missile defense could be used in first strike situations where we just don't want to be attacked when we get done killing a country.
Which the world has been fairly stable for the last ten to 13 years, America has not taken over to many countries (at least any of the ones anyone cares about). So I think we will be safe for the next 10 to 15 years also, from any major world wars. (Note the Defense shield would shut china up, and I know we would all like that. But also not that China is stubborn as a mule and would most likely just build up their weapons or try to build a defense shield of there own. Which they might be able to do it, if we sold them the technology for it.)