Conservative author and commentator Charles Krauthammer has died at the age of 68, two weeks after he announced that his abdominal cancer, which the Fox News commentator had been battling for more than a year, had returned.

Originally trained as a psychiatrist (he served as chief resident of the psychiatric consultation service at Massachusetts General Hospital), Krauthammer became one of the conservative movement’s most influential writers in the 1980s and ’90s, writing for the New Republic and the Weekly Standard, regularly appearing on Fox News, and writing a weekly column for the Washington Post beginning in 1985. He won the Pulitzer Prize for Commentary in 1987 for his work for the Washington Post.

Krauthammer’s conservatism was not simple to define — for example, he backed the invasion of Iraq in 2003 (he called it “The Three Week War”), but he was also a dedicated animal rights supporter, arguing in 2015, “One measure of human moral progress — amid and despite the savageries we visit upon each other — is how we treat the innocent in our care. And none are more innocent than [animals].”

His point of view was rooted in the idea that American foreign and domestic policy should be proactive, not reactive. But unlike many conservative pundits today, Krauthammer’s work was not linked to any one personality or administration, but to specific ideas to which he stayed loyal throughout his life.

I spoke to conservative commentator Ben Shapiro about Krauthammer’s legacy. He told me Krauthammer was “an incredible human being whose courage in the face of tragedy allowed him to become the most respected conservative advocate and analyst of his generation.”

Krauthammer’s writing career spanned four decades, beginning as a speechwriter for Democrat Walter Mondale during his 1980 presidential campaign. But during the Reagan presidency, Krauthammer wrote often and believed firmly in a muscular and outward-facing American foreign policy, one aimed at defeating communism wherever it lived and restoring American legitimacy abroad after the failure of Vietnam.

It was a foreign policy he eventually coined as the so-called “Reagan Doctrine” in a 1985 essay for Time magazine:

The Reagan Doctrine proclaims overt and unashamed American support for anti- Communist revolution. The grounds are justice, necessity and democratic tradition. Justice, said the President in his Feb. 16 radio address, because these revolutionaries are “fighting for an end to tyranny.” Necessity, said Secretary of State George Shultz in a subsequent address in San Francisco, because if these “freedom fighters” are defeated, their countries will be irrevocably lost behind an Iron Curtain of Soviet domination. And democratic tradition, said the President, because to support “our brothers” in revolution is to continue--”in Afghanistan, in Ethiopia, Cambodia, Angola ... (and) Nicaragua”--200 years of American support for “Simon Bolivar ... the Polish patriots, the French Resistance and others seeking freedom.”

Following the fall of the Soviet Union, Krauthammer argued that the United States should be the center of power in a reconfigured world. In 1990, he wrote in Foreign Affairs, “The immediate post-Cold War world is not multipolar. It is unipolar. The center of world power is the unchallenged superpower, the United States, attended by its Western allies,” but added that the enemy of such “unipolarity” wasn’t liberals, but conservative isolationists.

During the Bush administration, he argued that ”people are now coming out of the closet on the word ‘empire,’” because of then-President Bush’s own foreign policy efforts to remake the world in an American image. (For this, he was termed a “neoconservative” by others within the right.)

But Krauthammer’s own brand of conservatism was not easy to define (for example, he argued in 2005 that higher oil prices could encourage energy conservation), and certainly not one of Trumpism. He refused to vote for Donald Trump in 2016, writing in a column explaining his objections to Hillary Clinton:

A case so strong that, against any of a dozen possible GOP candidates, voting for her opponent would be a no-brainer. Against Donald Trump, however, it’s a dilemma. I will not vote for Hillary Clinton. But, as I’ve explained in these columns, I could never vote for Donald Trump.

And in 2017, he wrote that he had been convinced that Donald Trump Jr.’s emails regarding a meeting between the Trump campaign and a “Russian government attorney” — in which the president’s son replied “I love it” to the prospect of receiving potentially damaging information regarding Clinton — constituted collusion.

Once you’ve said “I’m in,” it makes no difference that the meeting was a bust, that the intermediary brought no such goods. What matters is what Donald Jr. thought going into the meeting, as well as Jared Kushner and then-campaign manager Paul Manafort, who were forwarded the correspondence, invited to the meeting, and attended. ... There is no statute against helping a foreign hostile power meddle in an American election. What Donald Jr. — and Kushner and Manafort — did may not be criminal. But it is not merely stupid. It is also deeply wrong, a fundamental violation of any code of civic honor

After an accident during his first year of medical school, Krauthammer spent the rest of his life using a wheelchair, a fact that he argued should not figure into how he would be remembered. “I thought that would be the worst, that would be the greatest defeat in my life — if I allowed that. I decided if I could make people judge me by the old standard, that would be a triumph and that’s what I try to do. It seemed to me the only way to live.”

In a letter to Washington Post readers regarding the return of his cancer, he wrote earlier this month: