So basically Trump is bluffing Korea as he has no authority to launch an attack on Korea without congressional approval. And it He persist can be impeached for not following the constitution.
Trump lacks the authority to order a preemptive strike on North Korea. While the president has a responsibility to defend the United States when another country attacks, the power to declare war (and, in so doing, to authorize the use of force) rests with Congress.
John Conyers and the other members of the House who have written Trump are obeying the oaths they swore “to support and defend the Constitution of the United States against all enemies, foreign and domestic.”
Donald Trump swore a parallel oath to “preserve, protect and defend the Constitution of the United States.” Congress should remind him of that by outlining clear limits on the use of force—especially with regard to to preemptive and unilateral nuclear strikes. If House Speaker Paul Ryan chooses to try to block action, the threat that his sycophantic relationship with the president should be recognized and addressed by the chamber.
If, in the end, Trump chooses to disregard his oath, then he should be impeached and removed from office.
Should it come to that, John Conyers will, again, be able to call on wisdom grounded in experience. He is, after all, the last remaining member of the House Judiciary Committee that voted for Articles of Impeachment against Richard Nixon in July 1974.