The Roswell Incident was a conspiracy theory which alleged that an alien spacecraft had crash-landed in Roswell, New Mexico on Earth in July 1947. (TOS - Strange New Worlds VI short story: "Whales Weep Not", TOS - The Eugenics Wars novel: The Rise and Fall of Khan Noonien Singh, Volume 2)
Unbeknownst to the general public, the incident was caused by a Ferengi shuttle called Quark's Treasure being accidentally sent back in time from 2372 as a result of a time warp. Three of its occupants, Quark, Rom and Nog, were taken into custody and transported to the Roswell Army Air Field, where they were examined, while their ship was stored in Hangar 18 of the facility. Constable Odo, who had stowed way aboard Quark's Treasure, disguised himself as a German Shepherd dog and infiltrated the base with the intention of rescuing the three Ferengi and returning to their own time.
U.S. President Harry S. Truman ordered General Rex Denning to find out all that he could from the three Ferengi. Denning was deeply suspicious of the Ferengi, whom he consistently referred to as "Martians," and believed that they were planning to invade Earth. His subordinate Captain James Wainwright agreed with this assessment of the ersatz Martians.
Seeking to create a new and better alternate timeline in which the Ferengi would be much more powerful and influential than in the original one, Quark attempted to open up trade for advanced technology with the United States government by negotiating with Denning. However, these attempts caused Denning to distrust Quark even more as his behaviour reminded him of his brother-in-law, a used car salesman. He was also concerned that Quark would make good on his idle threat to supply technology to the Soviet Union if the US government was not unwilling to trade.
After reporting back to President Truman, General Denning had the Ferengi detained for interrogation and ordered Wainwright to discover their true purpose in visiting Earth. However, Quark, Rom and Nog were able to escape with the assistance of Professor Jeffrey Carlson and his fiancé Nurse Faith Garland. While Denning and several MPs nearly apprehended the Ferengi, they were incapacitated by Odo. The four time travellers returned to their own time by recreating the time warp. Denning then told Captain Wainwright that they had nothing to worry about as they had found nothing more than a crashed weather balloon. (DS9 episode: "Little Green Men")
The Roswell Incident marked the first time that any Earth government had been provided with proof of extraterrestial life. In the wake of the incident, the US government established an organisation called Majestic 12. Its mandate was to collect evidence of extraterrestrial activity on Earth and develop strategies to combat alien invaders. (TOS novel: From History's Shadow)