Why being prepared for war is the best way to avoid it

Speak softly but carry a big stick.

It's a sad old world when people have to be prepared to kill other people.

Very true.

But since that's the reality, the question is how to survive in a 'sad old world'.

Running down our armed forces and making ourselves appear vulnerable to aggressors is likely to make our little patch of this world even sadder.

Putin is prepared to kill other people as much as we wish to avoid war.
The best way to avoid full blown war is by ensuring that there's no way, conventionally or otherwise, for him to be able to achieve anything from it.

It's a bit like a school bully beating up all the other kids and getting money from them.
As soon as that bully gets the good hiding of his life and knows it'll be even worse next time if it doesn't stop, it ends.

It's a sad old world as you put it.

I disagree. Had Ukraine been in NATO Putin would have left well alone. 

The problem is that we and even more so most other NATO members have gotten used to relying on the umbrella of American protection. As Trump and his followers are demonstrating, this is no longer a given. Democracy itself even in the USA is under threat as never before.

We are now in a world where we might have to be ready to fight without US backing. We and the rest of Europe need to be able to stand together as a strong force without the USA if necessary. Before anyone brings up the EU and Brexit, the free countries of Europe and ourselves cooperating in a military alliance does not require EU membership at all. Just NATO without the USA if necessary. And in any scenario where Europe and we alone have to stand up to Putin, the biggest aces up our sleaves are the independent nuclear arsenals of ourselves and the French. Like all sociopaths Putin cares not one jot about other people. But he values his own ass and knows it only takes one nuke landing in the vicinity of wherever he is hanging out and he is toast.

In the absence of credible backing from the USA, French and British nukes are the best deterrent we have. But we need the capacity to fight conventional wars too and ought to be stepping up our support for Ukraine - and by we I mean all of Europe and not just us - to the point where the USA is not needed anymore. Because US internal politics is making it an unreliable ally. One whose support cannot be taken for granted.