It was on October 31st, 2004, that Harry finally succeeded. He had won the Final War. The mighty task of killing the monstrosity known as Lord Voldemort, oops, make that Voldewhore, was complete. In the final battle at what was left of the Ministry for Magic, he had taken Riddle head-on. He had done that many times before, but this time he out-right won — he had killed Riddle with the Sword of Gryffindor. He had finally eliminated that abomination from the face of Earth.
It was a cadmean victory though.
Tom's insanity had dragged the Muggles into the Wizarding U.K.'s Pure-blood Civil War. Tom simply did not understand the power the Muggles had. His last real interaction with the Muggles had been the year before World War II ended, 1944. He had missed entirely the United States dropping the Atom Bombs on Japan in August 1945. If he had realized the damage just one of those could do, he never would have bothered the Muggles.
His failed Death Eater attack on the Prime Minister and Her Majesty the Queen at Buckingham Palace in 2000 had been an unmitigated disaster for the Wizards. Yes, one Wizard can kill dozens, or even hundreds, of Muggles, but even bees can swarm and kill a man. And the ratio of Muggles to Wizards in England was 600-to-one in 2000. Most Muggle-born could do the math, Tom Riddle, A.K.A. Lord Voldewhore, certainly hadn't bothered to do so.
The turning point was when the Muggle death toll from attacks reached 15,000 in one year. The Obliviators simply couldn't keep up, and that Voldewhore and the Death Eater's were in charge of the Ministry and the D.M.L.E.'s didn't help either. The Muggle military, with the help of a few Muggle-borns who had turned to the Muggle government after seeing their families killed at the instigation of the Ministry, started with ordinary tactics. However, the shields and protective enchantments the Wizards used demonstrated the impracticality of that approach in the first month. And few, read none, non-Muggle-born Wizards came to help the Muggles, so the Muggles quickly adopted a policy of annihilation instead of confrontation for all non-Muggle-born.
They started using cannon-fired tactical nukes on Wizard enclaves. No wizarding shield can stand up to the 10-million degree heat of a nuke. And while electronics fail in a magic field, few magic fields extend to the height necessary to affect an exploding nuclear bomb. A bomb capable of creating a crater a 500 yards in diameter when it explodes 300 yards in the air above a target creates a blast wave that goes through protective enchantments like a blow torch through a marshmallow. And unplottable only works if you haven't plotted everywhere else! Their orbital satellites certainly helped the Muggles in that regard. They didn't know what was in the blank spot on their charts that the computers said was there but they couldn't see, but the Queen's Government certainly didn't approve it. That made it fair game.
In a matter of a month, the U.K. Wizarding population plummeted fifty percent.
In desperation, the Wizards turned to the imperius, but the Muggle military was long used to enemy infiltration and quickly adopted measures to mitigate the attacks. And when you have isolated teams that only work via radio, and require two confirming sources for their orders, how do you get close enough to the decision maker to use the imperius? Especially when you have a clue-less Pure-blood whose understanding of passing as a Muggle was zero. A few times they did succeed, though, changing instructions or coordinates, and sent the nukes towards Muggle London or another large city. By the end of a year, the U.K. Wizarding population numbered less than ten thousand, and six million Muggles had perished. Which only hardened the Muggle's resolve to eradicate the Wizards.
By the second year, less than two thousand Wizards remained alive, while the Muggles had suffered another three million deaths.
And the blatant violations of the International Statute of Wizarding Secrecy brought the world's attention to the magical world. With the typical Wizard response being one of obliviation for the Muggles, the war spread rapidly. Obliviating a local Muggle witness doesn't help when someone kilometres away on the receiving end of a video camera saw what had happened. And that viewer saw the Wizard doing the obliviating as well.
Wizardkind was dead; the survivors just didn't know it. The shopping districts, the business districts, Hogwarts, the Ministry, Wizarding villages, the Quidditch stadiums — all provided gathering areas where Wizards and Witches could congregate and meet potential spouses. Those were now nothing more than smoking ruins.
The Magic gene is a small genetic variation affecting less than two-tenths of one percent of the world's population in the 20th Century. Without the support structure of magical villages, schools, shopping districts and other places Wizards and Witches could congregate, any Wizards or Witches that were born now would be unable to find a spouse to propagate the species. The situation was the same as it had been over a thousand years ago. Wizards and Witches had to wander the world to find others of their kind. Only today's world provided a near infinite number of ways where a Muggle could detect the Wizard or Witch hiding in their midst. And the ratio of Wizards to Muggles now was so much higher. A needle in a haystack was easy by comparison.