The issue of nuclear-armed-missile interception is not
widely discussed but it is important because it relates to two rogue states,
Iran and North Korea, that are a central part of contemporary global
politics. They are the big issue these
days.
We are already in the era of nuclear-armed-missile interception. Japan has reportedly stationed 53 launch-phase intercept missiles in the Japan Sea. The question of whether these missiles work is moot. One ICBM missile being launched in North Korea is a very bright object, starting very slowly and still not moving fast within the first few minutes when it could be hit by several Japanese intercept missiles. (Several would be fired to avoid the failure of any one of the interceptors.) The problem, I expect all of us to know about soon, is that any missile North Korea launches will be attacked unless North Korea agrees to inspections for all missiles, pre-launch.
The issue of final-phase nuclear-armed-missile interception
presents a different story. The two
countries most at risk from the rogue states are Israel and Japan. To understand the unique geographic issue,
picture cones stretching up from Israel’s population centers and from
Japan’s. The cone for incoming missiles
aimed by Iran at Israel is 75 miles in radius at the bottom (covering nearly
all of Israel) and only slightly larger fifteen miles up, outside the
atmosphere. 
Israel’s final-phase nuclear-armed-missile interception doesn’t have to be very good to wipe out all incoming nuclear-armed-missiles within a 75 mile radius cone above the atmosphere. Israel would use a nuclear weapon in defense, easily vaporizing an incoming missile, and the fallout would scatter over the planet. Something we would all suffer from.
The same is true for Japan. Japan can protect 80% of its population with four cones, 75 miles in radius and also not much larger fifteen miles up, outside the atmosphere. Few other countries have this type of missile defense feasibility: one known enemy, one direction for attack and a home population that is contained in a small area.
Japan has already committed the money to develop the final-phase intercepts and it should be online in less than five years. Nothing is being said about the use of nukes on the intercept warheads, but self-defense makes it imperative.
PostScript: It is doubtful that Iran or North Korea would risk developing a nuclear missile warhead without testing it. Measurements to detect a warhead test are presumeably effective and would give Israel and Japan plenty of time to prepare the final details of a defense.