evidence: Bloodshed and violent confrontations have occasionally occurred at the Koreas’ heavily fortified border, called the Demilitarized Zone. While Sunday’s incident happened amid simmering tensions between the two Koreas, observers say it is unlikely to develop into another source of animosity as South Korea believes the North Koreans did not deliberately commit the border intrusion and North Korea also did not return fire. At 12:30 p.m. on Sunday (11:30 p.m. Saturday ET), some North Korean soldiers who were engaged in unspecified work on the northern side of the border crossed the military demarcation line that bisects the two countries, South Korea’s Joint Chiefs of Staff said. [...] The 2155-mile-long, 2.5-mile-wide DMZ is the world’s most heavily armed border. An estimated 2 million mines are peppered inside and near the border, which is also guarded by barbed wire fences, tank traps and combat troops on both sides. It’s a legacy of the 1950-53 Korean War, which ended with an armistice, not a peace treaty.