WASHINGTON — Less than a week into Donald Trump’s presidency, he has already challenged American foreign policy since the end of the Cold War and resumed the Nixon administration’s war on the American news media.

Sparked as much by his oversized ego and need for self-gratification as by sober thought, Trump has shattered more than a half-century of rational behavior in a first week of reckless, impulsive actions that raise questions about his mental stability as a national and world leader.

His ludicrous concern over the size of his inaugural crowds and his loss of the 2016 popular vote seem to have set him off in a surge of peevishness more suited for a schoolboy than the president of the greatest country in the world.

Trump’s decision to order a federal investigation into widespread voter fraud he insists cost him that popular vote, along with his reaffirmation that he will build a wall across the Mexican border and make Mexico pay for it, has already triggered a major foreign-policy crisis.

Mexican President Enrique Pena Nieto has firmly reiterated his country will not pay for the wall and on Thursday canceled a scheduled meeting with Trump in Washington. Trump thereupon declared falsely that the decision was mutual, attempting to dodge the appearance of a rebuff to him.

The Mexican pushback comes on the heels of Trump’s vocal disparagement of the North Atlantic Treaty Organization and its European members, whom he has said have welshed on their obligations to pay their agreed-to share of NATO obligations. This bulwark of the Cold War remains at the heart of American-European postwar guarantees of peace and economic cooperation, which Russian President Vladimir Putin, Trump’s new mutual admirer, has sought to undermine.

As for Trump’s war with the American press, which has strenuously challenged many of his false declarations, the president’s chief strategist Stephen Bannon has added fuel to the fire by telling a New York Times reporter: “The media should be embarrassed and humiliated” for not foreseeing Trump’s election and should “keep its mouth shut and listen for a while.”

The former CEO of the so-called alt-right Breitbart News website called the news media “the opposition party.” In a sense, the remark was a dig at the Republican Party, under whose banner Trump sought and won the Oval Office. Bannon apparently telephoned the Times reporter to support Trump’s new press secretary, Sean Spicer, who caused flurry of journalistic criticism last week for lecturing the White House reporters, and for passing on Trump’s false claims about his crowds and other unproven contentions.

For older members of the Washington news media, the assaults from Trump, Bannon, Spicer and Trump aide Kellyanne Conway brought recollections of the antagonism generated in the Nixon years by the then-president himself and particularly by his vice president, Spiro T. Agnew. The vice president, who was later forced to resign amid evidence that he took cash payments from construction contractors while he was governor of Maryland and thereafter, reveled in becoming “a household name” with his alliterative verbal assaults on the press.

In a nationally televised speech in Des Moines in the fall of 1969, Agnew took direct and pointed aim at the television network news anchors for their “instant analysis and querulous criticism” of a Nixon speech. He argued that his boss had the right to speak “without having the president’s words and thought characterized through the prejudices of hostile critics before they can even be digested.”

He called them “this little group of men who not only enjoy a right of instant rebuttal” and asked, “What do Americans know of the men who wield this power?” All they knew, he said, was that they all lived in New York or Washington, and “worse, they talk constantly to each other, thereby providing artificial reinforcement to their shared viewpoints.”

Before long, Agnew became giddy with his alliterations like “nattering nabobs of negativism” and “pusillanimous pussyfooters” to define the press and enthrall his own followers. Trump, though, doesn’t resort to such fancy words or subtleties. The cruder the better, as he has often demonstrated.