So much for basketball diplomacy. Self-appointed ambassador Dennis Rodman’s trip to Pyongyang didn’t keep his new B.F.F., North Korean leader Kim Jong-un, from threatening a pre-emptive nuclear strike that would turn Washington into a “sea of flames.”

Rodman’s trip can be written off as narcissistic self-promotion. But Kim’s bombastic rhetoric shouldn’t be dismissed so easily. Pyongyang’s two deadly acts of war against South Korea in 2010 and its long history of terrorist acts show the regime often follows through on its threats.

A nuclear attack on the United States or full-scale invasion of South Korea remains highly unlikely, however, as either would ensure North Korea’s destruction. But it is only a matter of time before the regime launches another tactical-level attack on the South.

There is now a greater risk of miscalculation and escalation, due to new leaders in both Koreas. Kim Jong-un lacks experience and may stumble across red lines that his predecessors would have known not to cross. Moreover, he may be emboldened by North Korea’s new nuclear muscle and the knowledge that neither Washington nor Seoul ever responded to previous attacks.

Newly inaugurated South Korean President Park Geun-hye criticized her country’s past passivity and vowed to hit back hard and “exponentially” in case of another attack. The danger is that even a low-level retaliation could escalate into an all-out conflict. As a U.S. general on the peninsula warned, “Before you start even a limited response, you better be prepared to go all-in.”

Since repeated diplomatic efforts have failed to curb North Korea’s reckless behavior, the United States and its allies need strong military forces to protect themselves. Unfortunately, President Obama’s “Asia Pivot” was, itself, little more than rhetoric. Not a single unit will pivot from Afghanistan, Iraq or Europe into the Pacific. And massive defense budget cuts undermine U.S. military capabilities and credibility.

Seoul needs to improve its rudimentary missile defenses to better protect its cities and military bases. President Park should also implement her predecessor’s defense reform plans to upgrade South Korean forces and enhance response capabilities to North Korean attacks.

Prime Minister Shinzo Abe has already increased Japan’s defense expenditures, reversing an 11-year trend of ever smaller security budgets. He is also contemplating long overdue measures to improve Japanese defense capabilities. Though mistakenly referred to by the news media as Japanese "remilitarization," the measures simply remove self-imposed restrictions on Japan’s ability to defend allied forces under attack or rules of engagement when operating as part of U.N. peacekeeping forces abroad.

-Bruce Klingner is a senior research fellow for Northeast Asia at The Heritage Foundation. He previously served for 20 years in the C.I.A. and Defense Intelligence Agency, including as the C.I.A.’s deputy division chief for Korean analysis.