Some residents near the site of a controversial US anti-missile system and militant leftists have carried their demands too far, causing a national security concern.

In Seongju, North Gyeongsang Province, two launchers and a radar of the Terminal High Altitude Area Defense battery are running on emergency generators, as the high-voltage grid necessary to operate the radar has not been installed yet.

The US military tried to truck oil to the site, but couldn’t. Traffic to the site has been halted by local residents in a bid to stop the oil supply reaching the system they oppose. Fuel has since been airlifted in by helicopter.

An airlift would be time-consuming and inefficient. On May 21 when North Korea launched a ballistic missile, the radar reportedly became inoperative because the generator ran out of oil temporarily.

Police have warned the protestors not to stop traffic, but have not taken action. The Seongju county office has not ordered them to vacate the road.

In Uijeongbu, Gyeonggi Province, on June 10, a concert to commemorate the 100th anniversary of the foundation of the US Army 2nd Division stationed there was disrupted by anti-US activists.

Intimidated by threatening phone calls and trolls from opponents of the concert, singers either did not show up or apologized onstage instead of performing and left.

The reason they gave for opposing the event was that it was not appropriate to hold a concert just three days before the 15th anniversary of the deaths of two local teenage girls who were run over by an armored vehicle.

The deaths caused by careless driving during off-base training are deeply sad, but using an accident that happened 15 years ago as a pretext to scuttle the event to strengthen the Korea-US solidarity is hardly understandable.

The US Army 2nd Division is a symbolic unit of the Korea-US military alliance. It entered the 1950-1953 Korean War. During the war, 7,094 soldiers of the division died in action, and 16,575 were wounded. It is the only US combat division on the Korean Peninsula. The 2nd Division is scheduled to move to the newly constructed US base in Pyeongtaek, Gyeonggi Province, next year.

The THAAD was deployed at the US request to protect its troops from North Korean missiles, but South Korea as its host country receives collateral benefits that about half of its territory comes within its defense range.

The US forces beef up South Korean security as well as deter North Korean provocations. Nevertheless, progressive leftist groups disparage their value and try to instigate anti-American sentiment, while advocating the North Korean regime. They probe for chances to crack the Korea-US alliance. They pursue the out-of-touch goal of unifying with the North without intervention. They do not seem to care about free democracy and market capitalism.

Worse still, the Seoul-Washington ties are strained over the pace of the full deployment of the system. The new South Korean government decided to assess its environmental impact fastidiously, substantially postponing the installation of the remaining four launchers by about a year.

Under these circumstances, if thoughtless civic groups stir anti-Americanism, Washington cannot but fall out with Seoul. Their irresponsible incitement threatens the Korea-US alliance.

Scott Snyder, senior fellow and chief Korea analyst at the Council on Foreign Relations, said in an article in the Foreign Affairs magazine that South Korea could end up giving US President Donald Trump a pretext to pull American troops out of the country if it reverses the decision to host the THAAD.

South Korea relies on the US for much of its security. And yet, the new government and police do little to stop some groups’ rash acts. A lukewarm response to anti-THAAD and anti-US activities will only make Americans doubtful about South Korea’s commitment to the alliance.

President Moon Jae-in visited the South Korea-US Combined Forces Command on Tuesday. He wrote on the visitors’ book, “The Korea-US alliance is steadfast. Let’s go together.” Curbing anti-THAAD and anti-US activism is the good place to start.