The Karsids were a major space-faring civilization in the Alpha or Beta Quadrants, a species that gave rise to the Karsid Empire occupying territory that would later be part of the Federation and the Klingon Empire.
The standard pattern for Karsid expansion was to covertly make contact with more primitive societies, generally adopting the guise of ‘traders from another land’, and gradually introducing their technology and weapons until they had made the society completely dependent on the Karsids to maintain their standard of living. The Karsids did not reveal they were from off-world until their hold on the economy was all but unbreakable.
One world conquered in this fashion was the Klingon homeworld. Karsid first contact took place in 1486, and within a generation, Qo’noS had become part of the Karsid Empire, with induction to full tributary status taking place in 1540. Some Federation historians considered this event something of an object lesson in the Prime Directive; Klingon mercenaries, equipped with Karsid weapons and technology, would go on to lead revolts against the Empire, leading to its eventual collapse in the 19th century.
Another world targeted by the Karsids for conquest was Earth. Karsid automated probes began observing Earth in 1868, and the first Karsid agents made contact with Humans in 1873. However, the project met with failure, due to unexpected opposition by the United States government and American President Ulysses S. Grant. As their empire became wracked by rebellions soon after, there were no further attempts made to infiltrate Human society.
In 2267, Klingon agents travelled back in time in an attempt to alter the timeline to ensure the Karsid conquest of Earth would be a success. They failed; ironically, by attempting to alter history, they actually caused the event they sought to prevent. Their mission made several Humans aware of the existence of alien life, and thus able to later identify the Karsids as agents of a hostile extra-terrestrial power.