I won’t take credit for this idea. It was posted on r/worldbuilding but was nuked by the mods for “asking for content”, as usual only after lots of interesting discussion. Anyway…
Unbeatable aliens show up and blow up half the moon as a show of strength. They say that they’ll be back in six months to annihilate the Earth. For the next six months humanity lives in the shadow of their inevitable destruction, with all that mindset entails.
Then when the time finally comes, nothing. No earth shattering kaboom, no heroic last stand, the aliens just never show. What happens next? How do you rebuild?
It’s like your boss coming to you on 4:45 on Friday afternoon and telling you in a super serious tone “I need you in my office first thing Monday morning.” and then completely forgetting about it on Monday.
So, first, you have the question of what humanity does. You’d need a plan on how to respond immediately in order to keep some form of world order. You’ll likely see a division between two groups of people, those who believe they are working towards survival and those who don’t.
I don’t expect a unified response to survival, but I expect they will probably try to not stop on each other’s toes unless absolutely necessary. These are going to be the nations that raise armies and the manufacturing base necessary to go to war or flee.
To those populations who don’t see hope, I expect a rapid decent into chaos. Social order collapses and it is likely that those with a survival mentality have to soft land the chaos parts of the world. This likely includes raiding in order to take equipment and shut down dangerous facilities. Any defenders likely get co-opted by the survivor group.
At six months, it is likely that billions are dead in the chaos zones as their ability to sustain their populations collapse. The survivor groups are likely highly militarized and poised for a response that isn’t needed. I expect, after some debate, that the survivor groups further expand into the chaos lands for resources to feed their machine. The machine is still kept ready for a few years, a debate occurring as to the necessary readiness but still considered a necessity for the next few years.