This started as something in m' 'ead as the setup for like, a group of people who like, bust ghosts and other supernatural stuff. But it works as a Crow-Man mini-fic, so here we go.
At the end of World War 2, after the atomic bombings of Hiroshima and Nagasaki, the people there suffered and died for years to come. Now bear in mind that even in the modern day, there are people who genuinely believe in spirits and the supernatural; and that information was hard to come by in those days. Back then, the people there had no idea that it was the radioactive fallout that was the cause of their prolonged torment.
So while we as men of science might scoff at them now, it was with great earnesty that a group of high ranking officials arranged for and held a massive exorcism at ground zero of both Little Boy and Fat Man. The result, as we now know, was catastrophic.
Quite simply, the exorcisms failed and ripped the fabric of reality at both those sites, allowing all manner of malevolent forces to barge into our existence. The gods, roused by this, quickly mobilized their numbers, but caught off guard by this sudden influx of antagonistic spirits, they only managed a desperate stalemate that was fast disintegrating by the minute.
Only the sudden appearance of a powerful "Tengu", turned the tide at Hiroshima, allowing gods and men alike to repair the damaged existences there.
Nagasaki, faired less well. With Hiroshima secured, the gods and men there went to support their brethren. They pushed the invasion back, but the damage to reality there was too extensive, they could not repair it fully like they did at Hiroshima. The most they could do was seal the worst of it. And even then, several incredibly powerful entities had already come to our side and there was nothing that could be done about it.
Fortunately, as we've already mentioned, information did not travel freely back then. The entire disaster was covered up. What knowledge of events that escaped to the public, was dismissed as superstition. Science prevailed.
Of course, we now know who that mysterious "Tengu" was. It's obvious, now that we have retrospect on our side...