The soldier crossed the military demarcation line within the Demilitarized Zone and expressed an intent to defect to South Korea. The South Korean military tracked and monitored the soldier from the moment the person crossed the demarcation line.
The South Korean military's full-scale loudspeaker propaganda on all fronts may have affected the soldier. Loudspeaker propagation on the North can extend as far as 19 miles.
The South Korean military is currently evaluating the effectiveness of loudspeaker propagation on the North, which resumed after North's continued trash balloon dropping last month. North Korea in July resumed flying balloons likely carrying trash toward the South, adding to a bizarre psychological warfare campaign amid growing tensions between the war-divided rivals.
TV Chosun, a television network in South Korea, citing an anonymous government source, reported that the "Many of the recent defectors have said they changed their mind after listening to loudspeaker broadcasts at the border, and so did the man who defected on Tuesday."
The border area Goseong, which the North Korean soldier in his 20s crossed to defect, is a heavily surveilled area near the East shore. In the last few months, South Korea's military spotted North Korean laborers installing new mines around their side of the border inside the Demilitarized Zone several times. Experts say there's a high chance that he knew where to avoid if he was one of the soldiers mobilized to install mines.
What does the demarcation line look like?
The demarcation line, in many parts of the DMZ, is simply a sign mounted on a stick or a slice of concrete.
People have stepped across it before, under very special circumstances, and usually at the border village of Panmunjom. Former U.S. President Donald Trump walked across with Kim Jong Un. Last year an American soldier facing possible military discipline dashed over the line to the North.
Outside of Panmunjom, much of the DMZ is wilderness, but it is heavily monitored on both sides. And while the demarcation line may be easily crossed, it is very difficult to do so without being spotted immediately.
The southern side of the land border is protected not only by thousands of soldiers, guns and mines, but also by a dense network of cameras, motion sensors and other high-tech surveillance equipment. Breaches are very rare and are usually detected quickly. Defections from the North are also unusual along the North-South land border, though they have happened with frequency along the porous China-North Korea border and occasionally in the Yellow Sea.
The Koreas have had no meaningful talks for years and could find it difficult to set up dialogue as tensions rise over the North's development of nuclear-capable weapons.
Some analysts say the Koreas' poorly marked western sea boundary - site of skirmishes and attacks in past years - is more likely to be a crisis point than the land border.
Kim, during a fiery speech in January, reiterated that his country does not recognize the Northern Limit Line in the Yellow Sea, which was drawn up by the U.S.-led U.N. Command at the end of the war. North Korea insists on a boundary that encroaches deeply into South Korea-controlled waters.
While the huge military presence on both sides of the DMZ means that years sometimes pass without incident, violence can quickly erupt. Two American Army officers were axed to death in 1976 by North Korean soldiers, for instance.
The ABC News and the Associated Press contributed to this report.