North Korea's submarine nuclear ballistic missile capabilities are advancing. Time is running out.

On Tuesday, however, we got another complication. President Trump and Secretary of State Rex Tillerson offered distinctly mixed messages on North Korea.

Speaking to the State Department press corps, Tillerson stated in relation to North Korea, "We do not seek a regime change; we do not seek the collapse of the regime; we do not seek an accelerated reunification of the peninsula; we do not seek an excuse to send our military north of the 38th parallel. We're not your enemy, we're not your threat, but you're presenting an unacceptable threat to us, and we have to respond."

The commander-in-chief, however, preached a different message on Tuesday. Trump hinted at looming military action, commenting, "We will handle North Korea. We are gonna be able to handle them. It will be handled." It's part of a trend.

Last Saturday, Trump sent out two tweets addressing China's role in the North Korea crisis.

Trump continued,

And more news from last weekend. Sen. Lindsey Graham claims that Trump told him, "If there's going to be a war to stop [Kim Jong Un,] it will be over there. If thousands die, they're going to die over there. They're not going to die here." Graham, who sits on the Senate Armed Services Committee and is in a position to know, continued, "There is a military option to destroy North Korea's program and North Korea itself."

When Graham talks about destroying "North Korea itself," he is referencing the U.S. Military's OPLAN 8010. That base plan is the foundation for U.S. nuclear strike options.

Still, the key point here is that Trump has made increasingly overt suggestions that he will use force against North Korea. This message targets two corollary objectives.

First, as the Washington Examiner has suggested he should, Trump is building pressure on China. Unless and until China believes Trump will use force against North Korea, its leaders will not exert significant pressure on Kim Jong Un to suspend his ballistic missile program. China fears the prospect of U.S. strikes, worrying that they might destabilize China's southern border with North Korea.

Second, Trump is telling North Korea that he is ready to use military force. This is important because North Korea has traditionally succeeded in deterring U.S. presidents against the use of force. Altering that dynamic is crucial to produce a moment of pause in Kim Jong Un's calculation. Namely, his forced contemplation about whether the fruits of a ballistic missile victory are worth a nation of ashes.

Yet, when Tillerson equivocates on North Korea, or at least takes a more conciliatory tone, he dilutes the effect of Trump's threats. This is most pronounced in risking China's belief that Trump is bluffing.

Some might say that Tillerson and Trump are playing "a good cop, bad cop" strategy. Perhaps. But if they are, it's doomed to fail. In order to be successful, U.S. foreign policy requires a consistent and credible message. Absent that, our foreign policy is confused and weak. As I've argued, South Korea and the U.S. could successfully adopt a "good cop, bad cop" approach. But it has to be two different nations, not two different U.S. officials. Certainly not the two most senior officials on foreign policy.

Ultimately, Trump needs to get his team in order. North Korea represents the most significant threat to U.S. national security in many years. To address that threat diplomatically, Trump must show he is willing to use force if China and North Korea do not change their approach. But if Trump lacks the support of his cabinet, his credibility will perish, and diplomacy will fail.