Five topics President Trump and Rex Tillerson clashed over

Jessica Estepa | USA TODAY

Show Caption Hide Caption Rex Tillerson ousted by Trump via Twitter Secretary of State Rex Tillerson was fired on Twitter by President Trump after a 14-month tenure.

As secretary of State, Rex Tillerson often found himself at odds with President Trump — which can prove tricky when you're the nation's top diplomat.

More: Why Secretary of State Rex Tillerson was fired now and what it means for North Korea talks

Trump: Pompeo will be a great Secretary of State President Trump has "total confidence" in Mike Pompeo, his new pick for secretary of State. Trump announced Tuesday that he fired Rex Tillerson and would replace him with Pompeo, the CIA director. (March 13)

With Tillerson now on the outs with Trump, learn about five key topics that the two disagreed on:

Russia

Since he took office, President Trump has repeatedly expressed his doubts in Russian collusion in the 2016 presidential election.

On Monday night, after the House Intelligence Committee announced it was closing its Russia investigation, the president tweeted:

THE HOUSE INTELLIGENCE COMMITTEE HAS, AFTER A 14 MONTH LONG IN-DEPTH INVESTIGATION, FOUND NO EVIDENCE OF COLLUSION OR COORDINATION BETWEEN THE TRUMP CAMPAIGN AND RUSSIA TO INFLUENCE THE 2016 PRESIDENTIAL ELECTION. — Donald J. Trump (@realDonaldTrump) March 13, 2018

But Tillerson hasn't bought into that.

In December, during a closed-door meeting with diplomats, the secretary of State said Russia had interfered with the election. Further, last month, he said Russia was already interfering in the upcoming midterms.

More: Tillerson reportedly called Trump a 'moron.' Months later, he's out

North Korea

Trump's war of words with North Korean leader Kim Jong Un has been a pain point for Tillerson for months.

Last year, shortly after Trump dubbed Kim "Rocket Man" during a speech before the United Nations, the president appeared to undercut Tillerson for his diplomatic efforts to to resolve a nuclear standoff between the two countries.

“I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man,” Trump tweeted. “Save your energy Rex, we'll do what has to be done!”

Of course, now Trump has opened the door to an unprecedented face-to-face meeting with Kim. Tillerson had said he didn't believe that negotiations had advanced far enough for the meeting.

More: Secretary of State Rex Tillerson ousted in favor of CIA chief, Trump announces

Iran

Trump has long expressed his disapproval of the Iran nuclear agreement, which requires Iran to freeze its nuclear program for at least a decade in return for the lifting of sanctions. The president has called for a renegotiation of the deal, saying it had "disastrous flaws."

But Tillerson backed the United States staying in the deal. He has noted Iran has complied with the agreement, and pointed out the U.S. still had grievances with the country's actions.

Trump has cited their disagreement on the Iran deal as one of the reasons why he dismissed Tillerson.

"I thought it was terrible, I guess, he feels it was OK," he said.

Afghanistan

Last summer, Trump vowed to prevail over Taliban insurgents on the Afghanistan battlefield.

“Our troops will fight to win,” Trump said during a speech that didn't outline any specifics for ending the war.

Tillerson agreed the battle would be won — but over the negotiating table.

“This entire effort is intended to put pressure on the Taliban, to have the Taliban understand you will not win a battlefield victory,” he said.

Paris climate agreement

Last June, Trump opted to pull out of the historic climate change agreement aimed at curbing global emissions.

Tillerson pushed to stay in the accord, and he wasn't shy about discussing how he felt.

"My view didn't change," he said in the weeks after Trump's announcement.

Later, Tillerson expressed optimism that the U.S. may be able to continue its participation: "The president said he is open to finding those conditions where we can remain engaged with others on what we all agree is still a challenging issue."

Contributing: Oren Dorell