North and South Korea have been divided for more than 70 years, ever since the Korean Peninsula became an unexpected casualty of the escalating Cold War between two rival superpowers: the Soviet Union and the United States.

For centuries before the division, the peninsula was a single, unified Korea, ruled by generations of dynastic kingdoms. Occupied by Japan after the Russo-Japanese War in 1905 and formally annexed five years later, Korea chafed under Japanese colonial rule for 35 years—until the end of World War II, when its division into two nations began.

“The catalyzing incident is the decision that was made—really, without the Koreans involved—between the Soviet Union and the United States to divide Korea into two occupation zones,” says Michael Robinson, professor emeritus of East Asian Studies and History at Indiana University, who has written extensively on both modern Korea and its history.

In August 1945, the two allies “in name only” (as Robinson puts it) divided control over the Korean Peninsula. Over the next three years (1945-48), the Soviet Army and its proxies set up a communist regime in the area north of latitude 38˚ N, or the 38th parallel. South of that line, a military government was formed, supported directly by the United States.

While the Soviet policies were widely popular with the bulk of the North’s laborer and peasant population, most middle-class Koreans fled south of the 38th parallel, where the majority of the Korean population resides today. Meanwhile, the U.S.-supported regime in the South clearly favored anti-communist, rightist elements, according to Robinson.

“The ultimate objective was for the Soviet Union and the United States to leave, and let the Koreans figure it out,” he explains. “The trouble was that the Cold War intervened….And everything that was tried to create a middle ground or to try to reunify the peninsula is thwarted by both the Soviet Union and the United States not wanting to give in to the other.”

In 1948, the United States called for a United Nation-sponsored vote for all Koreans to determine the future of the peninsula. After the North refused to participate, the South formed its own government in Seoul, led by the strongly anti-communist Syngman Rhee.

The North responded in kind, installing the former communist guerrilla Kim Il Sung as the first premier of the Democratic People’s Republic of Korea (DPRK) in the capital of Pyongyang.

The Korean War (1950-53), which killed at least 2.5 million people, did little to resolve the question of which regime represented the “true” Korea. It did, however, firmly establish the United States as the permanent b__ête noire of North Korea, as the U.S. military bombed villages, towns and cities across the northern half of the peninsula.

“They leveled the country,” Robinson says. “They destroyed every city.” The armistice that ended that conflict in 1953 left the peninsula divided much as before, with a demilitarized zone (DMZ) running roughly along the 38th parallel.

Unlike another Cold War-era separation, between East and West Germany, there has been extremely little movement across the DMZ between North and South Korea since 1953. Robinson describes the border as “hermetically sealed,” which helps to explain the drastically different paths the two nations have taken, and the continuing divide between them.

With continuing strong ties to the West (and an ongoing U.S. military presence), South Korea developed a robust economy, and in recent decades has made steps toward becoming a fully democratic nation.

Meanwhile, North Korea remained an isolated “hermit kingdom”—particularly after the collapse of the Soviet bloc in the early 1990s—and economically underdeveloped, as well as a virtual police state ruled by a single family for three generations.

The North’s dedicated efforts to develop a nuclear program have also greatly heightened tensions with South Korea and its allies, particularly the United States.

Despite efforts at diplomacy under South Korea’s current president, Moon Jae-in, the stark differences between the two Koreas were on full display in the run-up to the 2018 Winter Olympic Games. Even as South Koreans began welcoming athletes from around the world to the Winter Games, Kim Jong Un’s regime in the North put on a military parade in Pyongyang’s historic Kim Il Sung square.

As CNN reported, four of the country’s missiles, the Hwasong-15, were on display in the parade as Kim watched from a balcony, then spoke about the evils of imperialism.

Appropriately, the parade commemorated the day Kim’s grandfather, Kim Il Sung, formed the Korean People’s Army (KPA) in 1948—a fateful year in the history of Korea’s division.

“Starting in 1948, there are two established state organizations run by Koreans, each claiming to be the legitimate leaders of the people of the whole nation,” Robinson says. “And frankly, nothing’s changed since then.”