Reports from the South Korean equivalent of the CIA suggest that North Korea may be preparing a new intercontinental ballistic missile launch. ICBMs are exactly the type of missile that North Korea would need to attack Hawaii, Alaska, or the United States mainland, so any such launch would be specifically designed to threaten the U.S.
This threat is likely why, following last weekend's North Korean nuclear test, Defense Secretary Jim Mattis warned North Korea against launching towards U.S. territories.
Regardless, that nuclear test has allowed North Korea to seize back the initiative from President Trump. And that's a problem because if the North Koreans set the development of the diplomatic and military dimensions of this crisis, we'll see continuing action from China and Russia to block or ignore U.S. diplomatic constriction of Kim Jong-un. China, in particular, will only pressure North Korea if it believes the U.S. will impose economic or military consequences absent that pressure.
Correspondingly, the U.S. must shoot down any ICBM that flies over Japan's home islands and remains in midcourse stage. Doing so would show U.S. willingness to escalate in defense of our interests, to repudiate Chinese and Russian demands for U.S. calm, and to prove the capability of our ballistic missile defense forces. In short, it would restore the U.S. strategic initiative in a calibrated way.
However, if Kim Jong-un now launches an ICBM without response, even if that ICBM is targeted at the middle of the Pacific (but not Hawaii), he will further consolidate the strategic narrative of this crisis. Trump's threats will be seen as paper thin, and North Korea will be close to establishing a status quo of nuclear weapons plus ICBMs. Considering the irrationality of Kim Jong-un's regime, that status quo cannot co-exist with U.S. national security.
This isn't a binary choice. Were Trump to launch a preemptive military strike on North Korea, he would risk a conflict before all diplomatic alternatives have been extinguished. Moreover, even if it failed to hit its target, were North Korea to launch an ICBM against a U.S. territory, Trump would need to respond with military force. These possibilities are realistic and deeply problematic; at their pinnacle, they represent the prospect of U.S. nuclear retaliation.
Shooting down a North Korean ICBM, if it comes to that, is thus the most proportionate and sensible U.S. response.