The Krolp were an alien species that arrived on Earth at the beginning of the 22nd century. The Krolp immediately attacked and overran most of the planet, conquering whole continents and killing humans indiscriminately. They plundered Earth's natural resources extensively. Conquered humans became Krolp subjects. They were given Krolp educations to make them more "Krolp-like". Humans so educated were often given bureaucratic positions within the territory held by the Krolp.
Krolp technology was far more advanced than anything human beings could conceive of. Even the basic theory of what made Krolp technology was beyond the reach of the most intelligent human scientist. The best any human could guess was that the Krolp manipulated gravity in as easily as humans manipulated electromagnetisim. Given the technological edge, an individual Krolp was very difficult to kill, and killing one Krolp usually cost a substantial amount of human resources. Their weapons could be used by humans, but not replicated.
Physicially, the Krolp were centauroids larger than an Earth horse. The Krolp were striped. Their heads were similar to a jack-o'-lantern in shape and appearance. The Krolp didn't have teeth so much as sharp, jagged jaw edges, and enormous eyes that glowed like a cat’s. They also had a stronger odor similar to Limburger cheese.
The Krolp announced their intentions by opening fire on the crowds that greeted them shortly after they landed. Various countries attempted to fight back, using hydrogen-bomb tipped warheads. Most were swatted out of the sky. Those that did hit Krolp ships did no damage. The United States and Canadamerged early on to pool resources, but even combined, the Krolp were more than a match for the two countries.
In a last ditch effort, the United States, Russia and China launched a coordinated uprising that killed 40-50 Krolp, but was put down completely. Then-President Harris Moffatt I was able to escape Washington, DC and head for the Rocky Mountains, eventually creating a rump U.S./Canada with its capital in Grand Junction, Colorado. For the next 50 years, the rump U.S. was allowed to exist by a treaty signed by both the free USA and the Krolp. Krolp who had committed some crime against their species became renegades and fled to the U.S.
In the mid-22nd century, the Krolp discovered deposits of silver in northeastern Utah, within U.S. territory. The Krolp signaled their intention to disregard the treaty, and demanded access to the silver. President Harris Moffatt III, knowing how destructive the Krolp's mining techniques were, refused, and attempted one last desparate uprising against the Krolp.
War did not begin immediately, however. Moffatt had plenty of opportunity to flee to Craig, Colorado, and initiate Plan Seventeen, which alerted human subjects in Krolp territory to begin acting. The Krolp Subgovernor of the South Central Region was taken to the hospital with an unknown illness (implicitly poisoned by one of the sympathetic humans in his staff). This soon spread to other Krolp officials and loyal humans, sometimes with fatal results. A Krolpish flyer went down, with more Krolp dead and injured. Bridges and overpasses also collapsed within Krolp territory.
In response, the Krolp began marching on the Rockies and Wasatch Range. Most of the USA's military power was centered around northeastern Utah, as a further deterent. After two days, the Krolp began to respond in earnest, after Govenror Vrank survived an assassination attempt. Grand Junction was destroyed. Almost immediately, Moffatt and his advisors moved further north, anticipating Craig would follow Grand Junction. By the third day, it was over; Krolpish forces earnestly began their drive into Utah, effectively routing all opposition, and mining shortly after taking the area. U.S. forces were overwhelmed and began surrendering in mass., Moffatt was exiled, and the Krolp mined the silver, their domination over North Americacomplete.