New York (CNN Business) Last month Shep Smith decided that he had simply had enough.

With President Trump actively distorting the truth and many of his own colleagues helping him do it, the Fox News star prided himself on anchoring a newscast that countered the network's pro-Trump opinion shows.

The way Smith saw it, he was making sure that accurate information was getting on Fox's air.

"I wonder," he told a Time magazine reporter last year , "if I stopped delivering the facts, what would go in its place in this place that is most watched, most listened, most viewed, most trusted? I don't know."

But he reached a breaking point. Sometime in September, according to a well-placed source, he went to Fox News management and asked to be let out of his long-term contract. Tensions with the opinion shows were getting to be too much.

In late September, 8 p.m. host Tucker Carlson mocked Smith for standing up for his friend and colleague Judge Andrew Napolitano after the judge was called a "fool" by one of Carlson's guests. The network's lack of a vocal defense for Smith following the incident bothered him and the whole episode factored into his decision to leave, according to a person familiar with the matter.

But that episode was just one of many skirmishes that weighed heavily on Smith.

Executives at the network leaned on him to stay, but to no avail. On Friday afternoon he announced his departure on the air, then exited the building immediately, clearly emotional about saying goodbye to his television home of twenty years.

Smith was a Fox News original. He didn't change over the years -- the network changed around him.

For months I have been working on a book about Fox News in the Trump age. Staffers have been confiding in me about the challenges of covering the news inside a network that is increasingly defined by sychophantic pro-Trump personalities like Sean Hannity.

Staffers on the news side unanimously point to Smith as a role model.

But "it was clear he wasn't happy, on air and off air," one of the staffers said after Friday's stunning resignation announcement.

Two other staffers also said he'd indicated he "wanted to leave" -- meaning that he was not forced out by management, as some outsiders immediately speculated on social media.

"I think it probably just got to be too much," one of Smith's allies inside Fox News headquarters said.

In my reporting, in the months before Smith's resignation, I have been asking sources about Smith and why he has decided to stay put at Fox while other top journalists have left.

"Some of the top names among the news side at Fox" have been "leaving voluntarily one by one," a former staffer pointed out, as big chunks of the network have basically been co-opted by Trump.

Carl Cameron, who used to be Fox's chief political correspondent, said earlier this year, "Shep and I were among the first hires" at Fox "and I give that man huge credit for continuing to do it. I reached my limit." He left soon after Trump took office.

Smith had an incredibly sweet gig, at least on paper. His most recent contract reached $15 million a year, according to a person familiar with the matter, far more than he would make at any other channel.

"Years ago, he told me it was all about the money," one of Smith's former colleagues said.

But in the Trump age, that former colleague said, it wasn't "about the money anymore. It's about saying he's holding down the mantle of journalism."

Another source challenged the suggestion that Smith ever cared primarily about the paycheck: "It's never been about the money for Shep. It's about the truth."

Smith's show was the ultimate manifestation of the tensions between news and opinion at Fox.

His 3 p.m. hour, "Shepard Smith Reporting," was one of Fox's most celebrated shows outside the building -- especially among critics of the right-wing prime time lineup.

But his ratings were low by Fox standards.

So on the one hand, Smith's fans were glad he was on Fox, trying to counteract the propagandistic shows elsewhere on the network's schedule. But they weren't part of Fox's core audience.

To the contrary, many of the network's loyal viewers detested Smith for the very same reasons that others cheered for his solid reporting. Trump was one of those loyal viewers who hated Smith.

"Shepard Smith Reporting" usually outrated CNN and MSNBC in its time slot, because Fox News as a whole has a big base audience, but his hour was the lowest-rated in Fox's daytime lineup.

Smith used to have a much-sought-after evening time slot, at 7 p.m., leading in to "The O'Reilly Factor." In 2013, he was moved to 3 p.m. and given a new title, managing editor of Fox's breaking news division.

In theory Smith would be in charge of anchoring big breaking news stories throughout the day, but this rarely happened in practice. Other hosts didn't want their time slots to be taken over by Smith. So he became relatively isolated in the afternoon.

And once Trump took over, Smith seemed to be marginalized. He was the target of occasional attacks from Trump, which was frustrating for Smith and his producing team, a source said.

In an interview with Time magazine when he renewed his contract in March 2018, he said the job has been "more challenging" during the Trump presidency, "and more challenging is more fun."

Time reporter Daniel D'Addario said that Smith "decided to stay" at the network "in some part because the times are so precarious. In his telling, before he signed his new contract, he was nervous about what would come on Fox's air after he left."

"To stop doing it would be bad," Smith said in the Time interview, "because I think that there is a need for it and I know the degree to which we care about it and focus on it and we want it to be as perfect as it can be. And I wonder, if I stopped delivering the facts, what would go in its place in this place that is most watched, most listened, most viewed, most trusted? I don't know."

Fox says a rotating set of anchors will take Smith's place in the short term. Long term, no one knows.

But Smith seems interested in being back in the public eye at some point.

"The decision to leave was Shep's and his alone -- he will be taking an extended period of time off to be with his family," Chris Giglio, a spokesperson for Smith, said. "Following that -- who knows -- he is not retiring."