Those technologies aren't for defense; every nation in the world throws loads of money to military expenses -including those countries who have zero chances of being ahead of any other- for a single reason: war is a very profitable business. Everything else is propaganda.
> Those technologies aren't for defense;
Have you ever heard of Mutually Assured Destruction? Basically the idea is having "credible deterrent" actually prevents the use of the weapons, because the side that makes a "single deadly upfront attack" will lose along with their opponent. The only winning move is not to play.
The problems happen when one side has capabilities the others lack, or when there are options for a "single deadly upfront attack" that aren't suicide.
In short, "offensive" technologies can be defensive.
To me also there's not that much difference between mutual assured destruction and severe damage. Let's say the US has 100 nukes and Russia only 20 (random numbers just to make an example), all of same power, and let's assume that most of them will be neutralized before hitting the targets so that in case of mutual attack Russia would be hit by say 30 nukes and the US by just 4. History 100 years from now would surely regard it as an epic victory for the US, but today anyone ruling over there would have to explain their citizens how on earth they couldn't take a step back and spare millions of lives.
It's probably much simpler than that. The US, Russia, China, North Korea, etc. have zero intentions to go at war with each other, but is in everyone's interest to keep fueling the weapons industry, so they keep playing the plot: citizens are hooked using nationalist arguments to make them supporting their rulers etc. Weapons keep being amassed until the day they become obsolete and can be sold to less rich countries for their smaller wars, new more advanced weapons take their spot and the business continues. It's a win-win scenario for everyone but common people.
The USA would never permit them to be invaded. We'd go to an all out war before we'd let a next-door neighbor fall to another country, and every nation knows this.
> though I hardly believe there aren't others in a civilized world.
Where is this civilized World of which you speak? I, sadly, see that the Globe is on fire.
https://en.wikipedia.org/wiki/List_of_ongoing_armed_conflict...
Of course war is a business, no question about that; however the world is a dangerous place, and humans are arguably the most unpredictable mammals alive.