Washington elites have long regarded war with North Korea as all but unthinkable. Yet, obscured by the wild twists and daily cacophony of the Trump presidency, the conventional wisdom is changing.

While President Donald Trump rattles sabers on Twitter and slams "Rocket Man" Kim Jong Un, there is also a perceptible hardening of tone among senior officials. Military action to halt North Korea's march to a missile tipped with a nuclear warhead that could hit the US mainland appears to be a growing possibility.