Given its potential to wipe out an aircraft carrier - for America a core projection of its power - no weapon under development in China exercises strategic imaginations in Washington quite like the anti-ship ballistic missile.
The US intelligence community, which is scrutinising China's military build-up ever more closely, knows less about the missile than any other aspect of the PLA's armoury. It throws up more questions than answers. Precisely how far has China's programme progressed? Would China ever really want to risk firing one, given the potential for disastrous miscalculation by its enemies?