I envisioned the alien situation as a half-life sort 3 sort of thing. The aliens, a small expeditionary force of the more humanoid and intelligent force show up, but rather than succeed in conquest they have their piecemeal beaten back by the underestimated forces of man, stranding a few of them on earth, as well as leaving a few unintentional introductions to the environment, in other words some strange-ass, alien nightmarish animals.
These animals do exceedingly well in the environment, for a time, taking advantage of mankind, being in shambles after the contact-war, but after man adapts to his new surroundings, they have a massive die-off, leaving the modern situation as follow. Mankind is a skeleton of its former glory, the world's nations being seperated into hundreds of thousands of loose confederacies of well entrenched city-states, the earth itself has taken quite a beating from the new species (alien animals and sentient humanoids) the massive CO2 from alien and human missile barrages that made up the contact war, and finally the nuclear zones, where alien's shot tactical nukes at mankind's strong points and where alien ships, being struck by mankind's nuclear stockpile have crash landed. The environment has adapted to the alien animals who have taken up there niches all over the food-chain, both on the top and bottom. The oceans are largely unaffected, with island nations being the most untouched on the planet. The stranded alien attack forces have also been building, hiding and plotting in the shadows.
The human confederacies have taken to constant warfare, trying to gain the upperhand and take valuable and scarce resources from competitors, using whatever they have available, whether it be tanks, helicopters, cavalry, or men with goddamn clubs. In some smaller countries that took the back burner during the contact war man has slowly recessed into the fuedal ages, with paranoid kings using primitive armies to secure his boundaries from the other, more aggressive confederacies outside his largely untouched land. These countries were of the third-world sort so they are basically on the same level technelogically and infrastructurally as the rest of the world, though the resources and untouched soil within their small countries are just as valuable as those outside. The world is in turmoil with internal and external threats abound, everyone, alien and man alike, trying to gain the upper-hand over one another, trying to become the most powerful of the warring factions, trying to . . . . . Survive!
At least thats how I envisioned it, the details can be reworked later, but thats how I see it going down.