US President Donald Trump’s conflicting remarks on North Korea and the controversy over which side should bear the cost for deploying a US missile shield system in South Korea demonstrate that our national security now faces a new challenge -- dealing with an unpredictable US leader.

The challenge comes the fact that Trump keeps throwing out inconsistent remarks on North Korea and its leader Kim Jong-un. His “maximum pressure and engagement” policy on the North’s nuclear and missile threats is also no less ambiguous than the “strategic patience” of the Barack Obama administration.

In his first months as US president, Trump tried -- and succeeded -- to make North Korea one of the top items on his agenda. His endeavors focused on putting pressure both on North Korea and its communist ally China.

The Trump administration’s pressure on North Korea and China was highlighted by the statement that “all options are on the table” -- an obvious reference to the possibility of military action like a preemptive strike. It raised tensions to the degree that talk of a possible war flourished.

But the new US approach unveiled after a government-wide review also underscored resolving the North Korean nuclear crisis through diplomatic means.

Granted, a strong-arm tactic could be a useful means to get the recalcitrant, wayward North back to the negotiation table. But Trump and his security and foreign policy aides have yet to offer a clear path for stopping the North’s nuclear and missile threats.

Then came the ruckus over the cost of the Terminal High Altitude Area Defense antimissile system. None other than Trump started it by openly saying South Korea should pick up the $1 billion tab for the system being installed to shoot down North Korean missiles.

In the face of an uproar in Seoul, Trump’s national security adviser played down on what his boss said, but, H.R. McMaster still mentioned the need to renegotiate the agreement that stipulates that South Korea provides the land and the US shoulders the operation cost for the THAAD battery.

This is a typical case in which Trump thinks of everything in line with his “America First” catchphrase. It is certain that Trump, who had accused US allies like South Korea, Japan and Germany of taking a free ride for security, wants to use the THAAD issue to demand Seoul pay more for the upkeep of the 28,000 American troops in South Korea.

In other words, Trump puts as much weight on financial calculations as on the strategic value of the South Korea-US alliance which for decades have contributed to upholding freedom and democracy in South Korea and the region.

Trump’s zigzagging on Kim Jong-un is also cause for concern. Over the past week alone, he provided strikingly different assessments of the North Korean leader.

In one interview, Trump said that Kim Jong-un was “very threatening” and that he says terrible things in an apparent reference to the 34-year-old dictator’s pursuit of nuclear arsenal and an intercontinental ballistic missile capable of hitting the mainland US.

“North Korea weighs on me, but we have to be prepared for the worst,” Trump said in the interview.

In another interview, Trump sounds like a totally different man: He said the North Korean leader was a “smart cookie,” noting that he secured power despite some adversities.

Trump went on to say that he was “absolutely willing” and would be “honored” to meet Kim.

By now, we are all familiar with this kind of flip-flop on Trump’s part -- during his campaign he once called Kim a “maniac” and then said he was willing to meet him over “hamburger.”

It is unsettling that we have to deal with a US leader who habitually resorts to bravado, bluster and loud talk and, more seriously, exhibits inconsistency and incoherence in a policy as critical as the one on North Korea. It is indeed challenging for us to simultaneously watch out for two men well known for their erraticism, impulsiveness and unpredictability.