President Donald Trump is, IMNSHO, misunderstanding the role of diplomacy in a shooting conflict. Secretary of State Rex Tillerson said,

He’s (Trump) not seeking to go to war.  He has made it clear to me to continue my diplomatic efforts…until the first bomb drops.

Of course, war should be a last resort, not a never resort, and Trump understands that.  But to say that diplomacy ends when the shooting starts, is mistaken.  Diplomacy doesn’t only shape the coming battlefield during a prior period of peace and during the runup to the fight.  It also shapes the battlefield during the fight: both directly with its (however minimal) impact on the enemy belligerents, but also on the periphery and the far field surrounding the battlefield through its impact on our allies, our enemy’s allies, and on neutrals.

It’s that diplomacy that affects the fight itself and, at least as importantly, sets the stage for post-fight actions.

Tillerson, again:

Rest assured that the Chinese are not confused in any way what the American policy towards North Korea (is) or what our actions and efforts are directed at[.]

Ongoing diplomacy maintains that clarity, as well as clarity of consequences, during and after the fight, both with our enemy’s allies (the People’s Republic of China in this case) and with our own allies (the Republic of Korea and Japan, for instance), as well as with neutrals like the EU, Vietnam, et al. (and make no mistake: the EU has shown itself neutral in the present matter of northern Korea with its own timidity of action).