Dear Prof,
You are quite correct, who knows what the future will bring.
What I do know.
I do know that the funding for Trident could be better utilised, I do know that defeating terrorism should be our top priority, I do know that our armed forces are not fully equipped, I do know that our police force and NHS struggles with under funding, I do know that the immigration crisis is not going away, I do know that we still have foodbanks.
We have problems now that need to be addressed, throwing money at a supposedly future threat does nothing to solve our very present problems.
Correct again, which is why the funding for Trident could be better utilised, better Army, Navy and Air Force and of course better communication with every country threatened with terrorism and for me, terrorism is a global problem.
Finally, has someone answered the question, why do we need four nuclear submarines?
Gonnagle.
I think this is a situation of either/or - we should be investing in all the things you suggest.
The point is that if we don't invest in replacement of trident we won't have a nuclear deterrent in 2030 onwards. We, of course cannot know what the future hold. However it is not unreasonable to consider that in 2030 to 2060 the situation may be such that the presence of an independent UK nuclear deterrent will be important and if we don't invest we won't have one.
I'm also rather sceptical about the focus on cost - certainly the headline figure of £40 billion looks eye watering, but you need to recognise that this may be spent over a 10 years period, so perhaps £4 billion per year. That would represent just 0.5% of public expenditure, so not proportionately a huge amount. Also this is money that is channeled back into the UK economy so its 'real' cost will be less, as a significant proportion will actually come back the government in tax revenues.
Now this doesn't change the fundamental argument about whether or not we should have a nuclear deterrent, but I actually think the argument against on cost grounds is a rather weak one.
Why four submarines - well this seems to be the minimum number that will guarantee that one is alway deployed. I'm not an expert, but that's what the experts do seem to think.
But the fundamental issue here is that any very major defence procurement takes decades, so you are always basing decisions on the unknown - in other words what the world situation will be in 20 or 30 years time. But just because we cannot be sure about that situation isn't a reason to do nothing.