And in the scenario the Visitors are defeated (which like you said, is probably not the most realistic), that serves as a factor for them turning on themselves.
Oh. Right. The Visitors by 2030 are giving the GPI a larger role in fighting the Resistance, given that although they feel it is becoming a problem, they feel the GPI is powerful enough to keep it under control. And the GPI spans the whole planet, thought the Visitors after that initial bombardment manage to take a significant chunk of the globe by the next day.
That chunk being most of Europe, Africa, the Western coastline of North America, and the Korean Peninsula, Brazil's coastline and the southern tip of South America, most of China's coastal cities, all of Tibet, Southern Japan, most of the Northeastern coast of North America, parts of the American Gulf Coast, Australia's eastern coastline, and Northern New Zealand.
Other major cities that have yet to fall are still in an intense period of brutal and bitter fighting, not to mention the occasional kinetic strike at the tactical level.
And humanity has scored victories against the Visitors during the Invasion but most of them were relatively minor and those were short-lived. The only major victories humanity managed to score were the First Battle of Xi'an and the First Battle of Detroit. Unfortunately for humanity, these two cities fell to the Visitors anyway.
Usually, the next round of fighting, would result either in the defenders getting slaughtered completely, though inflicting heavy losses in the process or being forced to retreat, but not without trying to inflict as many losses as they can.
At this point, though, humanity's military forces were on a fighting retreat, desperately trying to hold their ground for as long as possible before attempting to retreat to another defensive position.
And this happened until the end of the Invasion, when the surviving military forces were forced to flee into the countryside and begin the process of fighting a long, brutal and costly guerrilla war across the globe, and these national armies (or what was left of them) were among the first to organise what would later become the Resistance.
So, @FerretCivilization? Does this all sound clear to you?
I wonder how effective this could be in the objective of delaying the fall of Earth in a realistic scenario.
And those two battles I mentioned were not the only major victories humanity has had overall, but they were the only individual battles where we had scored a major victory, albeit at a terrible cost to human life and short-lived ones at that.