Come on, we, as in Dumbericans, today, would see this fleet coming as soon as it touched the helioshock. It's Alteria's fault for lacking the intellectual curiosity to build a telescope.
No, we as 'Merkins would never see this shit coming because we stopped spending anything on aerospace programs that weren't about spying on or exploding our enemies from orbit, or we would see the warnings from satellites and observatories and then ignore/obfuscate our coming doom in favor of more unbridled consumption, just as we always have.
"We had no prior knowledge of the attack."
"Could you please read the name of the report to the committee?"
"Gamilion Empire Set To Bomb the Fuckshitpigtits Out of Earth."