Could the DF-21D be converted into an ASBM that could be delivered by an aircraft or submarine?
The original DF-21 is actually a land-based version of the JL-1 submarine-launched ballistic missile, so I see no particular reason why the DF-21D couldn’t return to the sea in the form of a submarine-launched version (because of its maneuverable warhead, it might not fit into the single Xia-class submarine for which the JL-1 was designed, but that submarine is probably too old anyway and one of China’s newer Jin-class submarines could be modified to take it). In theory, I suppose it could also be launched from an airplane. The weight of the DF-21 exceeds the maximum bomb load of China's largest bomber, the H-6, but a transport aircraft such as an Il-76 could possibly be modified to carry it. In practice, however, you would probably want a different kind of missile, as the DF-21 is designed to be launched from a standing start at ground level, whereas an air-launched version would already have some altitude and velocity.
Another possibility would be to launch it from a surface ship. If it can be launched from underwater, it could definitely be launched from a surface ship, although it would have to be a specially-designed ship to accommodate the vertical launch of such a relatively large missile.
Overall, then, how much of a threat do you feel the DF-21D is to U.S. forces in the Pacific in combat conditions?
It’s impossible to know in advance just how effective such a missile is likely to be in actual combat. However, the mere possibility that such a system might be effective is likely to affect the way in which the U.S. would operate its surface ships in a crisis or conflict with China. If nothing else, it will force fleet missile defenses to split their attention between anti-ship cruise missiles flying along just above the surface of the ocean and ballistic missiles coming in from overhead.
My understanding is that an individual Aegis ship isn’t capable of simultaneously looking both for air-breathing threats, such as cruise missiles and aircraft, and for ballistic missile threats, so some of the ships would have to look for cruise missiles and aircraft while others looked for ballistic missiles. This decreases the number of ships available to defend against a given kind of threat and increases the likelihood that something will get through. The DF-21D isn’t necessarily a “game changer,” but it does add a dimension that wasn't there before. As we argued in our 2007 book, operating within about 1,000 miles of China’s coast is going to get increasingly risky due to a whole range of threats – aircraft, surface ships, submarines, and now ballistic missiles. A U.S. president or Pacific forces commander is going to think long and hard before he sends major surface ships into that area in a crisis and risks having the first aircraft carrier lost due to enemy action since World War II.
Roger Cliff is a senior political scientist at the RAND Corporation whose research focuses on security issues involving China. Recent studies have included Chinese air force doctrine, Chinese anti-access strategies, and China's aerospace industry. Cliff is currently leading a study on Chinese military space power. Before coming to RAND, Cliff served as assistant for strategy development in the office of the Deputy Assistant Secretary of Defense for Strategy and as a defense systems analyst for VERAC, Inc.