The public was woken up by a news alert all over the media.
War was coming...
The army had no intention of hiding this matter because it concerns everyone. The first thing they did was hold a press conference where they laid out the information they had gathered so far. They didn't downplay or overexaggerate the severity of the situation. Their words were concise and straight to the point.
War was coming. Their army was estimated to be 5 million at minimum; the weakest were, of course, Tier 1, but they had a Demon King at the helm of their operations. This was an unprecedented magnitude of a demon horde, one that could seriously threaten humanity's existence.