Krolp technology was far more advanced than anything human beings could conceive of. Even the basic theory of what made Krolp technology work was beyond the reach of the most intelligent human scientist. The best any human could guess was that the Krolp manipulated the strong and weak nuclear forces as well as gravity as easily as humans manipulated electromagnetism. 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 could not be replicated.
Physically, the Krolp were centauroids larger than an Earth horse. The Krolp had tiger-like stripes along their backs. 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 strong 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 Canada merged early on to pool resources, but even combined, the Krolp were more than a match for the two countries and easily defeated the alliance.
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 and the Wasatch Range, 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. along with smugglers from both species.
In the mid-22nd century, the Krolp discovered deposits of silver and a small amount of gold in northeastern Utah, within U.S. territory. The Krolp signalled their intention to disregard the treaty, and demanded access to the silver and gold. 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 the Wasatch Range. Most of the USA's military power was centered around northeastern Utah, as a further deterrent. After two days, the Krolp began to respond in earnest, after Governor 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 began their drive into Utah, effectively routing all opposition, and began their mining project shortly after taking the area. U.S. forces were overwhelmed and began surrendering in mass. Moffatt was captured in Montana, and sent to the Krolp's North American Capital of St. Louis, Missouri, and the Krolp mined the silver and gold, their domination over North America complete.