"The press takes Trump literally, but not seriously. Voters take him seriously, but not literally."

This, by political commentator Salena Zito, was the smartest thing written about the 2016 election and deserves a place in every dictionary of quotations.

Now let me give you some advice about General James Mattis, who has been named Mr Donald Trump's secretary of defence. Take him both literally and seriously.

Gen Mattis is a dictionary of quotes in his own right. I like the way he meets and greets. "Do not cross us. Because if you do, the survivors will write about what we do here for 10,000 years." Or: "I come in peace. I didn't bring artillery. But I'm pleading with you, with tears in my eyes: If you f*** with me, I'll kill you all."

With Gen Mattis, however, you get much more than just words. You get deeds. As the commander of the 1st Marine Division in the 2003 invasion of Iraq, he earned a daunting reputation as a master of kinetic warfare. During the push to Baghdad, he relieved a colonel of his command for not advancing fast enough. In 2007 he wrote, with General David Petraeus, the Counterinsurgency Field Manual, the template for the successful "surge" in Iraq. So fond of combat was Gen Mattis that the marines' affectionate nickname for him was "Mad Dog".

Full disclosure: Gen Jim Mattis and I are both fellows of Stanford University's Hoover Institution. I admire him and consider him a friend. In person, he is neither deranged nor canine, but softly spoken and erudite.

Gen Mattis is not only a fearsome warrior, he is a deep strategic thinker, a soldier-scholar in the mould of Emperor Marcus Aurelius, whose Meditations he carried with him in Iraq and Afghanistan.

You might have expected such an inspired appointment to receive a warm welcome. Dream on. The liberal media, humiliated by Mr Trump's triumph, have sought to misrepresent his transition as chaotic. As it happens, Gen Mattis' nom de guerre was "Chaos". In both cases, it is the kind of chaos you inflict on your enemies. Mr Trump is not re-enacting Celebrity Apprentice; he is assembling what would once have been called "a ministry of all the talents".

True, the choice of Gen Mattis is unorthodox. He will be the first general to run the Pentagon since Harry Truman appointed George Marshall in 1950. All other defence secretaries have been civilians, reflecting a longstanding American belief that "brass hats" need to be reined in by "frock coats". Mr Trump has decided to ignore that convention, as well as to ask Congress to waive the rule that a general must spend seven years in retirement before he can take the defence secretary job. Cue flapping frock coats. But I think this is the best decision Mr Trump has taken since his historic victory three weeks ago.

Not that the others have been bad. For treasury secretary, Mr Trump has nominated Mr Steven Mnuchin, who has been almost as successful financially as Gen Mattis has been militarily. Cue liberal lamentations that "Government Sachs" is back in business. But Mr Mnuchin left Goldman Sachs 14 years ago. Everything he has touched since then - from the movie Avatar to the failed housing lender IndyMac to the Trump campaign itself - has turned to gold. Ms Elaine Chao for transport is another smart choice. (It doesn't hurt that she's married to the Senate majority leader Mitch McConnell. Mr Trump needs to govern with Congress, not against it.)

Secretary of state is the toughest call and Mr Trump is right to weigh his options. The former presidential candidate Mitt Romney bad- mouthed him during the campaign and is on the record as a Russia- basher - hardly the ideal candidate when the No. 1 item on Mr Trump's foreign policy agenda is to do a "great deal" with President Vladimir Putin. As a strategist, Gen Petraeus is up there with Gen Mattis, but it may be too soon after his rap for unauthorised disclosure of classified documents, and another general might give the administration the look of a junta. The former mayor of New York Rudy Giuliani is a Trump loyalist who badly wants the job, but Senator Bob Corker looks the safer bet.

Yet, Gen Mattis at defence could prove to be the most important appointment of all. True, the secretary of state usually gets more headlines. But, as Mr Donald Rumsfeld proved (for better and for worse) under Mr George W. Bush, the man who runs the Pentagon has more resources by far at his disposal and can easily turn that military muscle into political power. It is in Mr Trump's interests to make Gen Mattis his right-hand man. Here's why.

As president, Mr Trump has the enticing opportunity to fix America's broken foreign policy. His deal with Mr Putin could end the war in Syria and resolve the not-so-frozen conflict in eastern Ukraine. A comparable deal with China could address the economic grievances of Middle America while creating a new basis for peaceful co-existence with the Middle Kingdom, addressing key flashpoints in the Asia-Pacific region such as North Korea, the South China Sea - and Taiwan.

The Trump presidency can also change the game in the Middle East by abandoning the Obama administration's ill-conceived tilt towards Iran. And it can jolt continental Europeans out of their complacency, so that Nato ceases to be an alliance paid for by Americans and taken for granted by Germans.

However, to achieve all this will require more than Kissingerian diplomatic skill; it will also need the credible threat of force - for without that, America's enemies and allies alike will take advantage of the businessman Trump just as they took advantage of the law professor Barack Obama.

This is where Gen Jim Mattis comes in. First, he has unrivalled credibility. It is not only marines who love the man. Even Ms Michele Flournoy, who would have had his job if Mrs Hillary Clinton had won, speaks of him with reverence.

Second, Gen Mattis is a hawk on Iran. Indeed, some say it was his readiness to contemplate military action against Iran that led to his being sacked from US Central Command by President Obama. He is unrepentant. In a lecture in April, he called the Teheran regime " the single most enduring threat to stability and peace in the Middle East".

Nevertheless, he argued against ripping up Mr Obama's nuclear deal with Iran. Gen Mattis will advise Mr Trump to keep the agreement, but to punish any future Iranian breaches of it with military retaliation. He will also propose tougher action against Iranian regional proxies, notably Hizbollah.

Third, unlike Mr Trump, Gen Mattis has no illusions about Mr Putin. He has spoken out against the Russian invasion of Crimea and eastern Ukraine in 2014, and has implicitly criticised the Obama administration for not being tough enough.

Finally, Gen Mattis has a playbook for the Chinese too. In his testimony before the Senate armed services committee last year, he stated that "efforts in the Pacific to keep positive relations with China" must be "paralleled by a policy to build the counterbalance if China continues to expand its bullying role in the South China Sea and elsewhere".

Theodore Roosevelt's mantra was to "speak softly and carry a big stick". Under Mr Obama, the US has lectured loudly and carried a limp twig. All that is about to change. Unlike Mr Trump, Gen Mattis speaks softly. And that big stick he carries is sharp too. Take him literally. Take him very, very seriously.

THE SUNDAY TIMES, LONDON