The verbosity of the post by Modern Elmo either misses the point or is so obtuse that I missed it.
When we got Polaris from the USA it was said that it had to have a white ensign on it but a target country would not be able to distinguish between a 'friendly' US missile or a UK missile. In other words we were firmly wedded to the US trinity.
A similar concern must attach to a conventional BM. Does it have a conventional or nuclear warhead? If it originates from a nuclear power, can you afford to sit back an wait to see whether it is nuclear or conventional?
If it be assumed that detrrence will not work the only other option is to eliminate the nuclear option before it is developed. To use a CBM once the target country has its own nuclear capability is to risk armagedon.