President Trump listens during a meeting in the Oval Office in April. (Evan Vucci/AP)

John Brennan served as director of the Central Intelligence Agency from March 2013 to January 2017.

My first visit to the Oval Office came in October 1990, when I was a 35-year-old CIA officer. Iraqi President Saddam Hussein had invaded Kuwait two months before, and President George H.W. Bush wanted to discuss the implications of a U.S.-led military coalition that would ultimately push the Iraqis out.

I remember the nervousness I felt when I entered that room and met a president of the United States for the first time. By the time the meeting ended, his intellectual curiosity, wisdom, affability and intense interest in finding the best policy course to protect and promote U.S. interests were abundantly evident.

Over the next quarter-century, I returned to the Oval Office several hundred times during the administrations of Presidents Bill Clinton, George W. Bush and Barack Obama. The jitters that accompanied my first Oval Office visit dissipated over time, but the respect, awe and admiration I held for the office of the presidency and the incumbents never waned. The presidents I directly served were not perfect, and I didn’t agree with all of their policy choices. But I never doubted that each treated their solemn responsibility to lead our nation with anything less than the seriousness, intellectual rigor and principles that it deserved. Many times, I heard them dismiss the political concerns of their advisers, saying, “I don’t care about my politics, it’s the right thing to do.”

The esteem with which I held the presidency was dealt a serious blow when Donald Trump took office. Almost immediately, I began to see a startling aberration from the remarkable, though human, presidents I had served. Mr. Trump’s lifelong preoccupation with aggrandizing himself seemed to intensify in office, and he quickly leveraged his 1600 Pennsylvania Ave. address and his Twitter handle to burnish his brand and misrepresent reality.

Presidents throughout the years have differed in their approaches to policy, based on political platforms, ideologies and individual beliefs. Mr. Trump, however, has shown highly abnormal behavior by lying routinely to the American people without compunction, intentionally fueling divisions in our country and actively working to degrade the imperfect but critical institutions that serve us.

Although appalling, those actions shouldn’t be surprising. As was the case throughout his business and entertainment careers, Mr. Trump charts his every move according to a calculus of how it will personally help or hurt him. His strategy is to undercut real, potential and perceived opponents; his focus is to win at all costs, irrespective of truth, ethics, decency and — many would argue — the law. His disparagement of institutions is designed to short-circuit legitimate law enforcement investigations, intelligence assessments and media challenges that threaten his interests. His fear of the special counsel’s work is especially palpable, as is his growing interest in destroying its mandate.

For more than three decades, I observed and analyzed the traits and tactics of corrupt, incompetent and narcissistic foreign officials who did whatever they thought was necessary to retain power. Exploiting the fears and concerns of their citizenry, these demagogues routinely relied on lies, deceit and suppression of political opposition to cast themselves as populist heroes and to mask self-serving priorities. By gaining control of intelligence and security services, stifling the independence of the judiciary and discrediting a free press, these authoritarian rulers followed a time-tested recipe for how to inhibit democracy’s development, retard individual freedoms and liberties, and reserve the spoils of corrupt governance for themselves and their ilk. It never dawned on me that we could face such a development in the United States.

On the international front, Mr. Trump pursues policies that are rooted in uninformed campaign promises, a determination to upend actions of his predecessors and an aversion to multilateral engagements. His ad hoc and frequently impulsive approach to national security is short-sighted and dangerous, as allies and partners are left uncertain about U.S. strategy and objectives.

The impact of the Trump presidency will be felt for many years to come. Most worrisome is that his use of falsehoods, his mean-spirited and malicious behavior, and his self-absorption will be emulated by many young Americans — indeed, young people globally — who look to the president of the United States as a role model.

The damage also will be felt by the millions of Americans who believe in Mr. Trump because of their concern about being left behind in a rapidly changing globalized world. These Americans have a legitimate gripe that politicians and political parties of all stripes have failed to deliver on the promise that America is the land of opportunity for all, irrespective of race, creed or place of residence. At a time when deep-seated fears of socioeconomic and cultural change need to be addressed honestly and without prejudice, Mr. Trump grandstands like a snake-oil salesman, squandering his formidable charisma and communication skills in favor of ego, selfishness and false promises.

Many have condemned my public criticism of Mr. Trump, arguing that as a former CIA director, I should bite my tongue. My criticisms, however, are not political; I have never been and will never be a partisan. I speak out for the simple reason that Mr. Trump is failing to live up to the standards that we should all expect of a president.

As someone who had the rare privilege of directly serving four presidents, I will continue to speak out loudly and critically until integrity, decency, wisdom — and maybe even some humility — return to the White House.