Nuclear 9/11
An accidental submarine run in, that's all it took. They happened to be nuclear subs, one American, one Russian. In a split second the world was immersed in nuclear war. The stockpiling began; the naive went for food and toilet paper, those more aware geiger counters or dosometers, and the batteries required to keep the functioning. The devices and batteries stored in every possible location to attempt assurance one would survive if they survived to use them. Most of the immediate deaths happened in the first forty eight hours. Missles were launched at all major cities and nuke stockpiles immediately when war was declared. Most were vaporized before they could clock what was happening, more still died of radiation burns so severe, their flesh was no longer connected to their bones. A small number suffered the slow, agonizing demise of radiation sickness. The length of their suffering diminished but worsened by the lack of medical care. These unlucky souls were caught up in the explosions at stockpile sites because these were the only ones that created substantial fallout.
That was it, the war was over. It ended faster than it started since the countries involved were immediately annihilated after years of stalemate.
What survivors there were sought sanctuary in less affected regions. Rural healthcare infrastructure was easily overwhelmed by survivors in desperate need of care they weren't equipped to provide. The extent of irradiated land is too vast to create monitered exclusion zones, especially since the government and economy are in shambles. Survivors hoard geiger counters to this day and pull them out when trecks to the old cities are undertaken in remembrance of the day forever etched into their minds. Country borders are used as checkpoints crossed only after radiological briefings. Permission is limited to survivors, ever since the decontamination efforts were abandoned. I've heard stories from survivors who have dared to return of the wildlife that thrives in the absence of humans. The former territories of the Russian Federation and the United States of America serve as vast reminders of what happens when countries have nuclear stockpiles.