Yes. All accidents (normally limited to those in the last 5 years) have to be declared: whether you were at fault or not.
Attach a note explaining the circumstances of the accident (if it wasn't your fault) and the insurer should not increase your premium (if it does, find another, more sensible, motor insurer). If you were at fault then it shows that you are a higher risk than many other road users.
Do not try to hide it. The insurance company will be happy to take your money (in the form of the premium payment), but when you first make a claim then they will do some investigating, starting with the national database of accidents. On finding your details they will declare your insurance invalid from the moment you took out the policy, thus opening you to criminal charges of driving whilst uninsured (amongst others).
If your accident in the white fleet vehicle involved only yourself or another military vehicle then the chances of you appearing on the national database are very remote (provided police did not attend the scene). If however you were involved in an accident with a civilian car then your name will certainly be on the database.
If you lie and are caught out then you will be screwed. Is a couple of hundred quid worth going to prison for, or loosing your job or being made bankrupt?
By the by, as of October next year most of the EU nations will start to share databases with each other. so if you have a crash in Germany, Spain, etc you will no longer be able to hide it from your UK insurer.
Same goes for speeding tickets and other penalty notices: the foreign police will soon be able to track you down and enforce payment.