Even before the creation of the humans, there have been several occurrences that can only be interpreted as attacks by demonic armies trying to stop the creation process. Inhuman sources tell of several armies that invaded our world in the course of many centuries.
The gate was opened, an army passed through, and the gate eventually closed again. The invading armies had no supplies and were finally destroyed. The demonic leaders made only rare appearances on the battlefield, probably summoned by their minions for a while. So contrary to common belief, it seems possible that a large demonic army enters our world if certain conditions are met. It should be our top priority to find out what conditions this may be.
Most of these invasions could be tracked to some unholy centre of magic. From there immense forces were evoked to open a gate through which the demonic armies entered our world.