Transcript for Defense Secretary signals differences with Trump in resignation

Let's turn to politics, the fallout growing from that bombshell resignation. Defense secretary James Mattis telling the president he is stepping down. Remember, this comes just a day after the president made the surprise announcement about withdrawing troops from Syria. Mattis has widely been seen as a stabilizing presence in the white house but now he's on his way out. Chief global affairs correspondent Martha Raddatz has more from Washington. Martha, good morning. Reporter: Good morning, Tom. This is the most consequential resignation from trump's cabinet yet. Jim Mattis brought vast experience, gravitas and a steady hand. He is adored by those who have served with him but he simply could no longer tolerate the differences he had with this president. The letter of resignation came just hours after secretary Mattis met with president trump at the white house. Mattis making perfectly clear he has deep foreign policy disagreements with the president, writing, my views on treating allies with respect and also being clear-eyed about both maligned actors and strategic competitors mentioning China and Russia specifically, are strongly held and informed by over four decades of immersion in these issues. We just do everything possible to advance an international order that is most conducive to our prosperity and values and we are strengthened in this effort by the solidarity of our alliances ending his letter with this, because you have the right to have a secretary of defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position. In the beginning, Donald Trump thought Jim Mattis was the perfect man for the job with the perfect nickname. Mad dog Mattis. Reporter: For Mattis who cringes at that nickname, it was duty first. Just weeks ago saying -- When the president of the United States asks you to do something in America you just do it. To quote Nike, just do it. Reporter: But Syria was the last straw. Mattis spent four decades there but Donald Trump despite Mattis' plea not to is pulling all troops out of Syria. Our boys, young women, men are all coming back now. Reporter: He is also planning to substantially reduce U.S. Troops in Afghanistan. The president had hinted that he and Mattis aren't always on the same page. I think he's sort of a Democrat if you want to know the truth. But general Mattis is a good guy. We get along very well. He may leave. I mean at some point everybody leaves. Reporter: But the reaction to Mattis' resignation was deep concern that his departure will leave the white house with an unpredictable president unchecked. Overnight majority leader Mitch Mcconnell who has been infamously careful to avoid criticizing the president releasing a statement reading, I am particularly distressed that he is resigning due to sharp differences with the president on these and other key aspects of America's global leadership. Putting him in sync with his democratic counterpart. Everything that indicates stability and strength, everything that indicates knowledge is leaving this administration. General Kelly, general Mattis, so many others. Mcmasters. Mcmasters, exactly. Tillerson. There is chaos now in this administration. As for that drawdown in Afghanistan that could begin soon as well. We currently have 14,000 troops there. Trump is expected to reduce that number by half, if not more. Mattis was not in favor of that either but we are told trump wants to fulfill a campaign promise that he believes is popular with his base, Cecilia. He wants to check these things off his list. You call this the most consequential departure. The concern I've heard from people over Mattis leaving is unlike any concern I've heard in any other resignation in this administration so far. He was really seen as a stabilizing force. Reporter: Absolutely, Cecilia. There is an unease at the Pentagon and abroad. I can't stress enough how much of a stability factor Mattis is seen as in the national security world and how much the troops admire him. He is staying on until the end of February, but clearly he will have very little influence. President trump said he will name a replacement soon, Cecilia. Okay, Martha, thanks.

This transcript has been automatically generated and may not be 100% accurate.