President Donald Trump said his decision to oust Secretary of State Rex Tillerson on Tuesday came down to disagreements over policy. “It was a different mind-set, a different thinking,” Trump told reporters at the White House.

The president said he and CIA Director Mike Pompeo, his choice to replace Tillerson, were on the same “wavelength.”

The Iran nuclear deal was one area of disagreement Trump singled out with Tillerson, a former oil executive who the president chose to serve as the nation’s top diplomat despite having no prior public sector experience.

But there were numerous policies on which Trump and Tillerson did not see eye to eye. Here is a look at some of the most contentious foreign policy and national security issues under Trump, and the positions both men took during Tillerson’s 14-month tenure atop the State Department.

IRAN NUCLEAR DEAL

Trump’s position: As a candidate, Trump criticized and promised to end the agreement, which eased economic sanctions on Iran in exchange for the country limiting its nuclear development program. Yet once he took office, Trump continued to recertify the deal — one of former President Barack Obama’s signature foreign policy achievements — but also continued to criticize it.

Then, last October, Trump announced that he would pull out of the agreement if Iran didn’t commit to fully ending its nuclear program. This move placed the decision in the hands of Congress, and Republicans chose not to place new sanctions on Iran, which would have effectively ended the agreement.

Tillerson’s position: Tillerson spoke in favor of keeping the Iran deal in place, a position he shared with Secretary of Defense Jim Mattis. Tillerson’s support for the agreement put him at odds with Trump on one of the president’s top foreign policy issues.

Just three days after Trump announced his move to end the deal, Tillerson told CNN that the United States planned to “stay in” the agreement. “We’re going to work with our European partners and allies to see if we can’t address these concerns,” Tillerson said at the time. His stance created another messaging problem for a White House that has often struggled to project a united front.

PARIS CLIMATE ACCORD

Trump’s position: Last June, Trump announced that the United States would pull out of the Paris climate agreement. The accord was signed in 2015 by nearly every country in the world, including the U.S. and China. It set new targets for reducing greenhouse gas emissions and was hailed as a significant step towards slowing global warming, though critics argued the emissions targets weren’t ambitious enough and have questioned if countries will meet their goals. Trump frequently criticized the agreement as a candidate. In his announcement pulling out of the deal last year, Trump said it was “very unfair at the highest level to the United States.”

Tillerson’s position: Tillerson expressed support for the Paris climate agreement during his Senate confirmation hearings, saying he believed in global warming and thought it required a “global response.” The comments generated headlines coming from Tillerson, who retired from his position as CEO of oil and gas giant ExxonMobil after Trump nominated him to serve as the secretary of state. After joining the Trump administration, Tillerson appeared to signal his interest in sticking with the Paris deal. Last September, two months after Trump’s decision, Tillerson said the U.S. would consider staying in the accord “under the right conditions.”

RUSSIAN MEDDLING IN U.S. ELECTIONS

Trump’s position: Trump has questioned news reports and the U.S. intelligence community’s conclusion that Russia meddled in the 2016 presidential election, calling the information “fake news” and a “hoax.” He has also criticized special counsel Robert Mueller’s investigation into Russian meddling in the election and potential ties to his campaign.

Tillerson’s position: Tillerson has taken a harder line on Russia than Trump. He has said that Russia’s election meddling poses a threat to the U.S. and other countries and told Fox News last month that Russia was already trying to interfere in the 2018 midterm elections. “I think it’s important we just continue to say to Russia, ‘Look, you think we don’t see what you’re doing. We do see it and you need to stop,” Tillerson said in the Fox interview.

On Monday, the day before he was fired, Tillerson said the poisoning of an ex-spy in Britain “clearly came from Russia” and “will trigger a response.” Trump was slower to embrace that hard line, though he said after a Tuesday phone call with British Prime Minister Theresa May that Russia needed to provide “unambiguous answers” about the incident, adding that whoever was responsible should face consequences.