Andrei Tarkovsky's "The Sacrifice": What Was Really Going On?

Okay, sci-fi and fantasy peeps, KevinMovies here. I know, I know, Tarkovsky usually isn't quite in our wheelhouse, but hear me out. I just watched "The Sacrifice" (again), and I'm still reeling. The atmosphere in this movie is just... palpable. The impending doom, the creeping dread... it's thick enough to cut with a Vibro-Knife. I am obsessed with world-building and love to unpack what makes a world or reality tick. This movie makes me think of the world of 'The Road' or 'A Canticle for Leibowitz' in that it's a world on the brink. However, unlike those titles, the 'end' isn't something to simply survive. It's something that is entirely about Alexander's inner-world/psyche. So, my question is this: Was the nuclear war ACTUALLY happening, or was it all in Alexander's head? He's already a pretty stressed-out dude even before the alarms go off. His relationship with his son is strained, he's clearly wrestling with some deep existential dread, and then BAM! End of the world. The whole thing feels symbolic, like a manifestation of his own anxieties. When he offers to give up everything – his voice, his home, his family – it feels less like a heroic sacrifice for humanity and more like a desperate bargain with his own inner demons. Thoughts? And bonus question: That final scene with Little Man planting the tree... What's the significance? Is it a glimmer of hope (planting seeds for a new beginning), or is it just a continuation of Alexander's madness being passed down? Also, I know Ingmar Bergman's cinematographer Sven Nykvist shot this, and the visuals are AMAZING. However, it's sad that it was Tarkovsky's last film. Makes you wonder what other worlds he could have given us if he'd lived longer. Just sayin'. This movie is seriously underappreciated and I believe would be a great additon to the Sci-Fi rotation.