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.