Moon Jae-in, the frontrunner in the presidential race, is zigzagging on the Korea-US agreement to deploy the Terminal High Altitude Area Defense battery, an advanced American antimissile system, on the Korean Peninsula.

The former leader of the main opposition Democratic Party said in a recent interview with a local newspaper, “I don’t think Korea can revoke what it agreed with the US so easily,” he said. “The next government should persuade China and Russia (regarding the agreement).”He also proposed a public debate and a parliamentary vote on the THAAD deployment.

He slammed the government shortly after it had announced the agreement last July, calling for the reconsideration and ratification of the deal. In October, he demanded the government stop all procedures for the deployment of THAAD and resume diplomatic efforts to scrap North Korean nuclear programs. In December, he argued that the current government should stop involving itself in the matter and leave it up to the next government.

Moon revealed his vague view of the THAAD deal in his essay book on his blueprint for South Korea, published Tuesday. “The decision on the THAAD battery had been made unilaterally by President Park Geun-hye. The issue should have been thoroughly reviewed at the National Assembly as it calls for parliamentary ratification.” He also wrote, “The issue should be reviewed in the context of the bigger picture instead of focusing only on North Korean nukes.”

Does he oppose or support THAAD deployment? What does the review in a bigger picture mean?

A presidential candidate’s position on national security is an important criterion for voters to consider in electing their president. Candidates should clarify their stances on such an important issue.

His about-face on the THAAD deal seems to show he has become prudent about the Korea-US alliance. But whether his latest remark will win public confidence is questionable. His position may change later again. Most political commentators see his flip-flop as a move to woo centrists and fend off expected criticism from conservatives who want the system on the peninsula. Singing different tunes for votes is the last thing a leader should do.

THAAD is almost like a symbol of the Korea-US alliance. If the next government is ambiguous on its deployment, Seoul will likely fall out with Washington, hurting the alliance and raising security risks on the Korean Peninsula. Trump may scale down or withdraw the US troop presence here, considering that he said during the election campaign he would be willing to withdraw the US troops stationed in South Korea unless Seoul paid a greater share of the cost of the US military deployment.

Moon is naive if he thinks he can persuade China and Russia easily. Democratic Party lawmakers have visited China twice so far over the THAAD issue, but they were far from persuading Beijing officials, who had proclaimed their opposition to the THAAD deployment on the Korean Peninsula. China and Russia are global superpowers, and China is North Korea’s most important ally and trade partner.

If Moon still believes there’s room for South Korea to play a balancing role in Northeast Asia, a role that the government under the late President Roh Moo-hyun wanted to take on, he might as well rethink. The vision has turned out to be unrealistic.

Moon said in the interview that the Kaesong Industrial Complex, where South Korean factories hired North Koreans, should be reopened and that South Koreans’ trips to the North Korean mountain of Kumgangsan should be resumed. He argued that South Korea could use the complex and the trip as leverage to influence Pyongyang. However, if the history is any guide, such leverage has little impact on the North Korean nukes. Rather, the money which flowed into the North is suspected of having been diverted to military purposes. The international community has tightened sanctions on North Korea, trying to block the inflow of funds there after it conducted nuclear and missile tests.

National security issues are directly connected to the survival of a nation. Moon, the leading presidential hopeful, should show voters his true colors on the THAAD agreement. It is candidates’ duty to provide voters with clear and accurate information about themselves.