In three words of a tweet this week, U.S. President-elect Donald Trump vowed North Korea would never test an intercontinental ballistic missile.

¡°It won¡¯t happen!¡± Trump wrote after North Korean leader Kim Jong Un said on Sunday his nuclear-capable country was close to testing an ICBM of a kind that could someday hit the United States.

Preventing such a test is far easier said than done, and Trump gave no indication of how he might roll back North Korea¡¯s weapons programs after he takes office on Jan. 20, something successive U.S. administrations, both Democratic and Republican, have failed to do.