Outgoing United States defence secretary James Mattis has the nickname “mad dog”, but for China and the rest of the world, he has been anything but during his two years in office. While he saw Beijing as his country’s biggest competitor, he also knew where to draw the line. That came from four decades of military professionalism and an appreciation of the US’ global role and place, factors that made him a cooler head in the midst of President Donald Trump’s chaotic decision-making and a brake on some of his unconventional instincts. He resigned because he felt too many of his views were at odds with those of the president; his departure at so sensitive a time for relations with Beijing and Asia raises the risk of even more turbulence.

Trump had praise for Mattis, but the defence chief’s letter of resignation was not reciprocal. It spoke of respecting allies and “being clear-eyed about both malign actors and strategic competitors”, obvious points of disagreement. His decision came a day after the president ordered the withdrawal of American troops from their fight against the Islamic State (IS) terrorist group in Syria and amid media reports of a hefty pull-out from Afghanistan, another issue of contention.

The president justified the pull-out of about 2,000 troops by contending IS had been defeated. Although the extremist group’s territory and fighting strength has been dramatically cut, it is still a global threat. To leave the battle now is to hand victory to an estimated 17,000 remaining fighters and open space for rejuvenation. The move also sends conflicting messages to Nato allies and Kurdish fighters, whose role on the ground supporting allied air power has been crucial to battleground victories.

At its peak in 2014, IS attracted tens of thousands of foreign fighters and controlled 8 million people in territory in northern Iraq and Syria the area of Britain. But that has been whittled down by 95 per cent and the group is now largely confined to a sliver of land along the Euphrates River between the two countries. Although weakened, many of its adherents have returned home with fighting skills and the threat persists, as shown by a recent Christmas market shooting in France and attacks against military forces in central Iraq.