Our history is a litany of violence, a shopping list of atrocities, mass murders, and other crimes against our own kind. The Planetary Citizens are a truly alien "Other", at least partly responsible for are current sorry state of affairs. In some areas they are actively taking over the planet, expanding their territory behind a wall of laser towers and missile batteries. It would seem that humanity would not only fight back, but do so with extraordinary zeal and no Geneva Convention to limit the possible tortures. There should simply be no more aliens
So the question has been occasionally posed - why are there still aliens? Shouldn't they have experienced the untold fury of the Human race and been wiped out within days of setting foot on the planet?
Logistics is a big part of it. Reanimates, disease, starvation, and internal conflict have taken a toll on our numbers. By some figures, if it was the EMP alone and the after affects of losing most of our food and energy distribution networks, a 30% population drop wouldn't have been unreasonable. Add reanimates and open warfare, and one understands why most estimates state the world has only five percent of the human numbers from before! Manpower is at a premium these days as well.
Nor can we count much on the inorganic parts of war. Munitions and Material are inadequate to equip a major army the size of the old national forces, and its unlikely that more weapons will be produced anytime soon. The experts needed to maintain or build the systems are gone or focusing on more immediate survival concerns.
Political disunity must be considered as well. Many of the remaining city states are hostile to one another. One of the most powerful remaining armed forces on earth is the viking like raiders comprised of remnants of the various Baltic navies, under the banner of the Kola Collective Admiralty. Even those on friendly terms are unlikely to send preciousness manpower hundreds of kilometers away to help against long term threats, when there is danger closer to home.
Since we never encountered hostile (or otherwise) space faring civilizations, we didn't have much in the way of anti-space-ship weapons. Of course there were anti-satellite missiles and lasers, but those are designed to eliminate objects that are intentionally built in the flimsiest manner possible as every kilogram saved is hundred of dollars saved in launch fees. Using these against an 800 meter long alien vessel is rather akin to trying to sink the Queen Elizabeth VII cruise ship with a hunting rifle. Nuclear weapons were not a real option either. Between treaties against nukes in space, and ICBMs are already amazingly complex and expensive weapons, giving them the ability to hit targets in orbit or beyond would have been a ludicrous technical and financial undertaking.
Hence, retaliation is still an issue. Their remaining ships are may be capable of deploying orbital weapons. Even a large rock dropped from lunar orbit would carry the power of an atomic bomb. This also means they have a way of quickly moving and concentrating forces when we do not. Sub-orbital jaunts could be the ultimate blitzkrieg.
One can't discount the difficulty of fighting them on the ground either. Summer blockbuster movies seem to make out all alien races as having a fatal flaw, and mankind's ingenuity and plucky heroes win out in the end. However, realistic thinking is that if they're advanced enough to build space ship, they probably have at least gotten to the late 20th century military wise. Indeed, while lasers in atmosphere are out ranged by mass-drivers, and none of their vehicles were heavily armored by our standards, they do have actual tactics, camouflage, guided missiles, aircraft, and other non-line of sight weapons. Although the laser towers have a fairly short range (if you consider a dozen km short - some estimates give them up to a several hundred megawatt power source) solid state phased array lasers can fire fast enough to shoot down incoming missiles..
For that matter, we have little proof that this was a significant fraction of their fleet. We don't know when or if more are going to arrive. Pissing off a few hundred thousand aliens when billions of reinforcements may be on the way - probably not a great idea.
Turning from the practical to the philosophical - we should keep in mind that the PCs are thinking sentient creatures. Depending on your school of thought, it could be argued they have a natural right to exist, or have established such a right through their society and technology. It is quite possible that they can be reasoned with and that we can work together against our problems. Most of them are quite willing to accept an attitude of live and let live, and with so much suffering on our part already - starting a war seems like a bad idea. It would also be far easier to reverse engineer their technology if some of the orginal builders are around to explain it.