On Monday, CNN reported "highly unusual" North Korean submarine activity. Put simply, Kim Jong Un's regime is deploying more submarines in different ways and on longer missions.

This is not too much of a surprise. North Korea has aggressively focused on submarine development and operability for the last five years. What's different now is that the regime is reaching a new level of submarine competence. And that matters for a few reasons.

First, it represents a new era. For a long time, North Korea's submarine fleet has relied on archaic Russian vessels from the early Cold War era. Those ships are now four generations out of date and easily detectable by even the most basic anti-submarine sensors.

In recent years, however, North Korea has embraced self-made submarines. These ships are far-inferior to their U.S., South Korean, or Japanese counterparts, but they have been mass produced. As such, the North Koreans might be able to overwhelm individual allied vessels by their sheer numbers. Consider that Kim Jong Un already has around 70-90 submarines. As he builds and deploys more, he will strain allied monitoring efforts.

After all, while the U.S. Navy has approximately 30 attack submarines in the Pacific (though some are always in maintenance), the South Koreans have only about 15, and Japan around 17. Over 6,000 personnel are on each U.S. aircraft carrier, so it's a pretty big problem if even just one North Korean submarine gets through.

That said, what's most concerning for U.S. security is North Korea's new "Sinpo-class" ballistic missile submarines (SSBNs). Based on Russian ballistic missile submarines, the Sinpo-class carry North Korea to the next level of submarine warfare. The challenge posed by SSBNs is their disruption of an adversary's confidence in detecting and destroying nuclear weapons before they can be used. While the U.S. tracks North Korean submarines, sometimes, as in 2010 when a North Korean submarine sank a South Korean corvette, they slip through the net. The Sinpo appears likely to provide a nuclear ballistic missile capability (SSBN) within 3 to 4 years, but it may be sooner.

One final problem? Based on North Korea's recent and rapid improvements in its land-based ballistic missiles, the U.S. cannot take anything for granted. As we saw last week, North Korean ballistic missiles can likely already strike the outskirts of Chicago.

Ultimately, this is just another wake-up call. The threat posed by North Korea is immense and it is growing. In turn, the Trump administration must prepare to strike North Korean ballistic missile development and combat forces. Absent that, it will never be able to get China to apply adequate pressure to Kim Jong Un.

Regardless, we're running out of time. Resting on a patient posture of missile defense is not an option.