Signs & Portents...Edit
The first indications Humanity had that other, intelligent, life existed was detection of faint radio transmissions by Quantum Interferometry, which operated by reading the Quantum companion-effects of a radio wave, operating instantaneously across distance.
While 2-way communication wasn't possible given the apparent age of the signals, the signals did prove conclusively that Humanity was Not Alone in the Galaxy.
The news was not greeted warmly by many, and triggered immense upset in the Religious Community, leading to terrorist strikes against Radio Telescope Arrays and the deaths of Researchers.
In the Military corridors, plans were quietly made to prepare in the off-chance Humanity did encounter intelligent star-faring Life that might perhaps prove hostile.
However, the Alien Threat Humanity feared, came not from the stars, but Humanity's own hand...the Chimerans.