then there are unknown unknowns
I have mentioned on these threads before Frankopan's The Silk Roads, and how it struck me reading it, that countries went and took their eye off the ball, and something apparently completely out of the blue came and finished them off.
Except, if our Government is anything to go by, it was probably more like complacency. Taking current events - pandemics were predicted, we could have been much better prepared generally, with PPE and effective tracing and quarantining systems. As for Russia and Ukraine - we could see what was happening in Crimea, but our Government was too fond of Russian oligarchs money.... Then Brexit - where any old deal is a good deal as long as it's nothing to do with the EU.