Recently there has been a lot of talk about China's new DF-21D Anti-Ship Ballistic Missile (ASBM) and, that it is a threat to US naval forces throughout the Pacific Ocean. However the Chinese have had many problems with high-end technology as evidenced by their lack of a modern AWACS aircraft and the fact that they have had a multitude of problems in developing indigenous weapon systems, without copying another countries systems with no licence
The missile system that the DF-21D is based off of is the DF-21, (DF standing for DongFeng). The DF-21 is an Intermediate Range Ballistic Missile (IRBM) capable of ranges between 1700-3000 kilometers depending on the variant. The DF-21's NATO designation is CSS-6 with the DF-21's variants having Mod 1-4. The DF-21 series has been in service since the early 1990s but, only now has turned into a real threat.
The method in which the Chinese have turned the DF-21D into a threat is through maneuverable reentry vehicles or MARVs. The difference between conventional multiple independently targetable reentry vehicle or MIRVs and MARVs is that a MARV is capable of inflight maneuvers thus, posing a greater challenge to anti-ballistic missile systems like the Aegis BMD system (Ballistic Missile Defense). One of the reasons ASBMs are not usually used by most countries armed forces is that ASBMs are extremly complicated. Imagine firing a model rocket at a moving bullseye 2-3cm wide and a meter long, 350m away and hitting the bullseye. This is probably not the best analogy but, it shows the challenges involed with this type of weapon system. In fact ASBMs are so complicated that the Soviet Union developed a sub fired ASBM designated the SS-NX-13 but, a couple of years later gave up on the system.
This is just an observers opinion but, given time the DF-21D could possibly develop into a threat.