(CNN) — President Donald Trump announced on Twitter Monday that he has fired Secretary of Defense Mark Esper, and that Christopher Miller, who serves as director of the National Counterterrorism Center, will become acting secretary “effective immediately.”

The President jettisoned Esper two days after his Democratic opponent Joe Biden was projected as the winner of the presidential election, a conclusion that Trump has refused to accept.

Esper’s increasingly tense relationship with Trump led him to prepare a letter of resignation weeks ago, an attempt to fashion a graceful exit in the widely expected event that the President decided to fire him, several defense sources, including one senior defense official, told CNN.

Miller, who will step into Esper’s shoes, has been a driving force behind some of Trump’s policies targeting Iran and its proxy group Hezbollah, as well as counterterrorism efforts linked to the wars in Syria and Iraq. Prior to heading the NCTC, Miller was director of counterterrorism at the National Security Council.

An Iowa native and retired US Army officer, Miller has also served as a deputy assistant secretary of defense.

Esper had been on shaky ground with the White House for months and had been pushing back on Trump since 2019 — important context for today’s firing, senior administration officials told CNN.

Esper was among those administration officials in 2019 who urged the President to release aid to Ukraine that Trump blocked as he pressured the country’s President for investigations into Biden’s son, Hunter Biden, and Ukraine’s alleged support for Hillary Clinton in the 2016 election. And Esper cautioned Trump not to completely pull troops out of northern Syria in October 2019.

After National Guard troops and US Park Police violently dispersed peaceful protestors in June, so Trump could hold a photo op in Lafayette Square, Esper took extra caution to try to keep the US military out of the realm of the President’s political desires.

Esper’s rift with the President deepened after he said in a June press briefing that he did not support using active-duty troops to quell the large-scale protests across the United States triggered by the death of George Floyd at the hands of police.

Esper, who did not want US troops in the streets during the unrest over the summer, told the President not to invoke the Insurrection Act, and in recent months was subjected to White House ire over his insistence that the Pentagon and US military not be part of the President’s reelection campaign, in contrast to the actions of Attorney General William Barr, Director of National Intelligence John Ratcliffe, Secretary of State Mike Pompeo, and Agriculture Secretary Sonny Purdue.

Esper’s June remarks from the Pentagon briefing room were seen by many as an effort to distance himself from Trump’s threats to deploy the military to enforce order on American city streets and went over poorly at the White House, multiple people familiar with the matter said.

According to multiple administration officials, White House sentiment about Esper had been souring for some time, with both Trump and national security adviser Robert O’Brien viewing him as not entirely committed to the President’s vision for the military.

For months, Trump and O’Brien had been frustrated by Esper’s tendency to avoid offering a full-throated defense of the President or his policies, the administration officials said.

One administration source told CNN that Trump had no respect for Esper, leaving the defense secretary with little influence and little choice but to take his lead from Secretary of State Mike Pompeo.

Trump dubbed him ‘Yesper’

Trump went as far as to mock his defense chief’s derisive nickname of “Yesper” during a news conference in August, a moniker bestowed upon Esper by defense officials who believed he did not go far enough in standing up to the President’s more controversial decisions.

Privately, Trump had expressed frustrations about Esper for months, venting about him at length during a trip to Camp David earlier this year, according to multiple sources.

Trump also publicly lambasted Pentagon leadership in September, accusing them of seeking to fight wars in order to boost the profits of defense contractors. Esper, angry, called White House chief of staff Mark Meadows to complain, according to defense officials. Meadows soon afterward appeared on television and attempted to walk back Trump’s comments, saying his broadside against the Pentagon’s leaders had not been directed at anyone in particular.

Esper and Trump also differed over the highly charged issue of whether to rename military bases that honor Confederate generals. Esper supported consideration of the renaming. The President refused to accept the idea.

The Senate voted 90-8 in July 2019 to confirm Esper, making him Trump’s second Senate-confirmed secretary of defense. He followed James Mattis, who had resigned in December 2018 over Trump’s decision to retreat from Syria amid the fight with ISIS, abandoning Kurdish allies and pulling US troops out of the war-torn country.

This story is breaking and will be updated.

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