Thanks for the kind words, @Bard Simpson, and my apologies for not saying so earlier. I don't know how I managed not to see your comment when you first posted it...
I'm a fair bit older than you, so I'm not sure the threat seems greater nowadays than it did when I wrote it in 1965 (I wasn't in my twenties, by the way, but still a teenager).
I'll admit that the world certainly still seems to be under as much threat now as it did then. But to my mind the current threat is climate change, and the apparent unwillingness, despite all the evidence, of the majority of individuals and countries to take any effective action to tackle it.