A study done in the 1970s investigated the environmental impacts of a nuclear exchange and concluded that six warheads hitting the US would basically kill everyone in a few months.
winds kicking up radioactive dust
water poisoning
Kills crops and livestock
food delivery stops
you can't purify the water it's fucked
you can't eat wildlife it's fucked
radiation fucks with gestational processes so surviving creatures can't procreate
but don't forget we have industrial systems everywhere that are now unmanned
pollutants get into the water
nuclear plants start to melt down
more water poisoning
entire middle of country becomes a gigantic cloud of radioactive toxic smog
like the Dust Bowl but 100000% shittier
But you'll say
we did all those tests
Yes. In the middle of fucking nowhere where they would have virtually zero impact on life. Remember how the twin towers falling down caused this miasma of glass and asbestos to rain down on people in NY? That could be smelled as far east as Montauk. Now imagine something three million times bigger and it happens in six different places, and you have no first responders because they are all dead, and you can't bury the corpses because there's millions of them and the people who bury corpses are dead too. Except it wouldn't just be six places. It would be 1200. Everyone you know and love would be dead in the first wave or by poisoning from the fallout.
The entire "post apocalyptic" zeitgeist in movies is a psyop to get you complacent with the nuclear arsenal because you get tricked into thinking that you're a special little snowflake who would somehow survive in that insane scenario. "Oh I know karate and bought a gun. I'm gonna shoot my way to a purified aquifer that doesn't exist!" It's a fucking fantasy.