After serving in World War II, Kirby returned to comics and worked in a variety of genres. He produced work for a number of publishers, including DC, Harvey Comics, Hillman Periodicals and Crestwood Publications, where he and Simon created the genre of romance comics. He and Simon also launched their own short-lived comic company, Mainline Publications. Kirby ultimately found himself at Timely's 1950s iteration, Atlas Comics, soon to become Marvel. There, in the 1960s, he and writer-editor Stan Lee co-created many of Marvel's major characters, including the Fantastic Four, the X-Men, and the Hulk. Despite the high sales and critical acclaim of the Lee-Kirby titles, however, Kirby felt treated unfairly, and left the company in 1970 for rival DC.