US President Donald Trump’s insistence that South Korea should pay for a newly deployed US anti-missile system threatens to overturn agreements and breaks faith between the two countries.

Trump said in an interview with the Washington Times on Friday, “Why should we pay for it? … I respectfully say that I think it would be appropriate if they paid for it.”

His remark came a day after he made similar comments for the first time in another interview with Reuters. “I informed South Korea it would be appropriate if they paid. It’s a billion-dollar system,” he told Reuters.

He gave the interview with the Washington Times after the South Korean government said that the US is supposed to shoulder the cost of the Terminal High Altitude Area Defense deployment in accordance with status of forces agreements.

Since it began THAAD negotiations in February last year, the Korean Ministry of National Defense has said that under the agreements, South Korea would provide the land and infrastructure for the system while the US would shoulder the cost of its deployment, operation and maintenance.

Trump’s demand breaks these agreements. He said he had informed South Korea, but the South Korean government said it has not been informed.

The missile shield is US property, brought here primarily to protect US forces in South Korea from the North’s missile attacks.

Although the system covers the southern part of the nation, the demand for it was made by US forces in South Korea in 2014. They wanted to use it to intercept 1,000-kilometer range Rodong missiles in case North Korea launches them at high angles to hit US bases in South Korea.

A THAAD battery is priced at around $1 billion, about 2.5 percent of South Korea’s defense budget for this year. If South Korea pays $1 billion, it would be as good as purchasing the battery. Trump’s comments contrast to the US government’s position thus far that it has no intention to sell THAAD to South Korea.

His demand does not only ignore US-South Korea accords on the THAAD deployment, but gives the impression that he places the dollar value of the weapon over the spirit of the alliance.

The value of the alliance cannot be calculated. An alliance is not one way but mutually beneficial. His breach of the THAAD deal threatens the foundation of the alliance, which defends free democracy.

Despite strong opposition from China and backlash from domestic civic groups and residents around the THAAD site, the South Korean government decided to host the system. It was a decision that considered the importance of the alliance as well as the military usefulness of the missile shield.

South Korea is already paying dearly for the deployment of the system.

National unity was deeply divided over it. The country is also suffering huge losses from economic retaliation by China, which opposes THAAD. Lotte, the corporation that offered the land for the THAAD installation, has seen most of its businesses suspended in China.

Trump’s insistence that South Korea should foot the bill for the weapon is bad manners to an ally that has already made economic sacrifices to host it.

Accusations that South Korea is free riding on the US for defense are ill-informed.

Last year, South Korea contributed 941.1 billion won ($825 million), about half of the overall costs of stationing US forces here.

It is also shouldering most of the construction costs for a new large US base in Pyeongtaek, Gyeonggi Province. The country is one of the biggest buyers of US weapons.

The US actually gets strategic and tactical benefits through its alliance with South Korea.

The alliance has lasted more than 60 years since 1953, when the Korean War was suspended in a truce. It is priceless and essential to keeping peace and security in Korea and Northeast Asia.

Adopting a money-focused, short-term view on the alliance may cause cracks and foster anti-THAAD and anti-American sentiment here.

For an air-tight alliance, Trump should withdraw his demand.