There were theories that Kim, who had taken control of North Korea's ruling dynasty in 2011 and is still only in his thirties, was concerned about a potential assassination or a coup at home if he left his country.
But then eagle-eyed reporters from Japanese television spotted a distinctive green and yellow armored train pulling into Beijing on Monday. After days of worldwide media speculation, the train pulled away. Only then, did North Korean and Chinese state offer confirmation: Kim had met Xi.
For his first foreign trip as North Korean leader, Kim Jong Un had followed a blueprint put together by his father, Kim Jong Il.
It wasn't until late May 2000 that Kim Jong Il traveled abroad - heading to Beijing to visit then-Chinese leader Jiang Zemin.
The similarities between this trip and Kim Jong Un's 2018 trip are hard to miss: in both cases, the Kims visited for three days; in both cases, they arrived unannounced on a train; in both cases, they met with the Chinese president and toured Beijing's technology hub in Zhongguancun; and in both cases, their visit to China took place ahead of a planned summit with South Korea.
The secrecy surrounding Kim Jong Un's visit to Beijing this week also echoed that of his father's various trips abroad, with state media only announcing the visit after it was over, presumably for security reasons.
Kim Jong Un traveled using his father's favored private train. According to a 2009 report in the South Korean press, the train consists of around 90 armored carriages, with two separate trains traveling ahead and behind it to handle security. Due to its weight, it moves slowly - its average speed is reported to be just 37 miles per hour - but inside, it is relatively hi-tech and luxurious.
Of the Kims, North Korean founder Kim Il Sung traveled most widely while he was the national leader. His longest trip came in 1984, when he used an armored train to travel through the Soviet Union and visit Eastern European countries - his son, Kim Jong Il, is believed to have accompanied him for at least some of that trip, likely traveling under an assumed name.
Since taking power in 2011, Kim Jong Un had remained in North Korea and has overseen a number of reported purges, even ordering the execution of his own uncle, Jang Song Thaek, in 2013.
North Korea expert Van Jackson suggested a trip outside the country could now show the young leader had gained confidence. "If Kim were in Beijing, it would be a strong indication that he believed he's successfully coup-proofed the regime for now," wrote Jackson, a former Pentagon official who now teaches at Victoria University in New Zealand, ahead of the confirmation that Kim had indeed visited China.
As surprising as Kim's visit to Beijing was in the short term, it fit into an established North Korean tradition. That will be something for American leaders and others to consider about Kim Jong Un in the future: He may be a millennial, but there are some Kim family traditions he wants to uphold.
(Adam Taylor writes about foreign affairs for The Washington Post. Originally from London, he studied at the University of Manchester and Columbia University.)
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