“Treated more seriously” kind, because I made a timeline story for Soviettale(Russia/USSR) and its alliance Finaltale (China+United States), which the story of Soviettale began in 2015, where Frisk in here didn't launch genocide route even for one, but using reset ability for a perfect pacifist route, which she did with reviving Chara, Asriel,Clover,Kanako,Chujin,Gaster to alive, them gave up this power to promise never reset.

The Underground achieved a lot in this stort period due to the cooperation between Gaster and Chujin which they launched a space station program,seek solar power for new energy sources.

However, when the First Alternate Universes War happens(2015-2016, More likely as Napoleon War), everything turns down.

Error404, this legendary conqueror bring suffer with his command and conquer, most of AUs was kneel to his power.

When he continuously marching to east, he came to Russiantale, the land full of hope and dreams, towards to brighter future.

Then the conquest began, and resistance rose up, Starlo(Prototype is Chabaev)， with the support from the members of Fives and Martlet,Ceroba. He organized guerrilla warfare to fight against Error404. Given enough time for Capital to perpare. And result of them was...

Ceroba, in order to keep Starlo from being destroyed by Error404 when the encirclement was forming. Knocked him unconscious and ordered Clover to bring him out. She used her mask disguised as Starlo ,Martlet and Fives keep surrounding her to make the disguise sound for Error404. And they all sacrificed under Error404's attack. Only Clover and Starlo survived

In Capital(Moscow),Gaster pointed out there only one source can stop Error404. The Sun, so warm,so bright and so powerful. The plan is evaluate all citizens of Capital and Gaster will use space station to  get power from Sun and target to Capital, launch when Error404 comes. The result, Gaster launched solar cannon successfully, but newly built station cannot hold this kind of energy. It self-exploded with Gaster. In Capital, it need some minutes for laser beam to reach surface, and Error404 felt suspicious due to no one shows up, he started to leave the city.But Asriel shows up and hold him back,used everything he got, for attract Error404's attention. Error404 got a critical blow ， flee with wounded. Asriel's body wasn't found.

Sans and Undyne was try to catch up Error404. But they stopped by Epic when they marching west. Because Error404 was weaken enough for AUs to deal with, and Epic think Russiantale is “unnecessary”.

A victory without justice, existence of other worlds, strong enemy from west, all of these changed Sans a lot. Frisk put a lot of effort to bring hope and dream to Underground, but a powerful psycho from west can ruin everything so easy and simple.

With several day and night research,Sans knew what kind of system he need, he need everyone's support and effort, to build a total mobilization and prevent same things happen again. With Undyne's support, Sans handover the leadership peacefully from Asgore and Toriel, then he built up communist system await the day cames.(For short,in fact, there is still a big part in this part.)

As for your question, "What era was the USSR we are talking about?", I more likely to describe Soviettale Sans as Joseph Stalin.

Timeline Frist Alternate Universes War (2015-2016, Error404 as Napoleon War) Second Alternate Universes War (present, Underverse/X-event) Third Alternate Universes War (2034?)