Once the basic technology for intercepting a ship at sea has been developed, however, it would be a relatively simple matter to install it on a longer-range missile. The DF-31A is also a road-mobile, solid-fuel missile, and has a range of 11,200+ kilometers, so China clearly has the capability to produce a road-mobile, solid-fuel missile of any range up to at least 11,200 kilometers. Three thousand kilometers might be a nice choice, as that would reach all the way out of Guam. Some modifications to the ASBM version’s maneuvering warhead might be required because of the higher reentry speed of a longer-range missile, but such adjustments would probably be relatively minor.
In the current U.S. arsenal of countermeasures, do you feel the U.S. has something that would be able to defend against such a missile? Would the SM-3 aboard Aegis vessels in the Pacific be able to provide a defense without their ever being a full test that U.S. officials could observe or study?
The U.S. arsenal has a variety of potential countermeasures, some of which I probably don’t even know about. The thing to keep in mind is that, in order for China to successfully attack a U.S. navy ship with a ballistic missile, it must first detect the ship, identify it as a U.S. warship of a type that it wishes to attack (e.g., an aircraft carrier), acquire a precise enough measurement of its location that a missile can be launched at it (i.e., a one-hour old satellite photograph is probably useless, as the ship could be 25 miles away from where it was when the picture was taken), and then provide mid-course updates to the missile. Finally, the warhead must lock onto and home in on the ship.
This complicated “kill chain” provides a number of opportunities to defeat the attack. For example, over-the-horizon radars used to detect ships can be jammed, spoofed, or destroyed; smoke and other obscurants can be deployed when an imagery satellite, which follows a predictable orbit, is passing over a formation of ships; the mid-course updates can be jammed; and when the missile locks on to the target its seeker can be jammed or spoofed. Actually intercepting the missile is probably the most difficult thing to do. The SM-3 has an exoatmospheric kill vehicle, meaning that it can only intercept the missile during mid-course, when it’s traveling through space, so an Aegis ship escorting the target would have to fire its SM-3 almost immediately in order to intercept the missile before it reentered the atmosphere, or else there would have to be an Aegis ship positioned right under the flight path of the missile. The DF-21D may be equipped with decoys that are deployed in mid-course, making the SM-3’s job harder. U.S. Aegis ships are also equipped with the SM-2 Block 4 missile, which is capable of intercepting missiles within the atmosphere, but the DF-21D warhead will be performing some high-G maneuvers, which may make it impossible for the SM-2 Block 4 to successfully intercept it.
How all this would work in reality is impossible to know in advance. Even after China has tested its missile against an actual ship, it won’t have tested it against one employing the full range of countermeasures that a U.S. ship would throw at it and, as you say, the U.S. Navy will never have tested its defenses against such an attack. Somebody is likely to be surprised and disappointed, but there is no way of knowing who.
Do you feel at any point China could or would attempt to sell versions of the DF-21D to other nations such as Pakistan or export such a system with less of a range?
The missile by itself would be pretty useless. As implied by my response to the previous question, an entire “system of systems” is needed to make it work. Some countries might buy them just to impress their neighbors, but their combat effectiveness would be negligible unless the country also invested in the needed detection, data processing, and communications systems. In any case, I doubt if China would sell it in the near-term, as that could result in knowledge of its technical details (e.g., radar frequencies and waveforms, etc.) getting out, making it easier for countries like the United States to figure out ways to defeat it.