A&E's Biography explains that Kim Jong-il came to power as a direct result of his father, Kim il-sung, designating him as his successor. Kim Il-sung's power over his country and people was essentially absolute, and his word was therefore effectively law when it came to his son's ascendancy. The elder Kim helped ensure his son's rise to power by giving him many influential and powerful positions to solidify his control.
After the Korean War, Kim Il-sung built a cult of personality around himself, making himself the unquestioned "Great Leader" of the country. He began grooming his son for leadership early on, putting him in control of the Propaganda and Agitation department of the Workers' Party. Between 1970 and 1980, Kim Jong-il occupied several positions of increasing power. By 1980, a personality cult began to form around him as the Great Leader's successor. He gained control of the Politburo, the Secretariat and the Military Commission, giving him control over much of the party's control apparatus. Once he gained control of the military, Kim Jong-il became the Supreme Commander of the Korean Peoples' Army, which ensured his ascendancy. Upon his father's death, Kim abolished the position of president, taking instead the titles of General Secretary of the Workers' Party and Chairman of the National Defense Commission.