Transcript for Fallout continues over Trump's surprise decision to pull troops out of Syria

Now to the fresh fallout on president trump's decision to pull U.S. Troops out of Syria. Brett Mcgurk has resigned. It comes just days after defense secretary Jim Mattis announced his departure. ABC's Lana Zak now on the latest. Reporter: Tonight, new backlash from the president's surprise announcement to pull all U.S. Troops from Syria. Resigning in protest, the top U.S. Envoy in the fight against ISIS, Brett Mcgurk, writing in an e-mail he determined, quote, I could not carry out these new instructions and maintain my integrity. Just 11 days ago, Mcgurk balked at the suggestion that the U.S. Could declare victory over ISIS. It would be reckless if we were just to say, well, the physical caliphate's defeated so we can just leave now. I think anyone who has looked at a conflict like this would agree with that. Reporter: But president trump doubled down on his decision tweeting today, "When I became president, ISIS was going wild. Now, ISIS is largely defeated." Today's announcement by Mcgurk comes after the resignation of secretary of defense James Mattis. In his resignation letter, Mattis wrote, "We cannot protect our interests without maintaining strong alliances." The example that Mattis specifically mentions in that letter? Mcgurk's defeat-isis coalition. Tom, Mcgurk will leave his post officially in February. Still no word who will replace general Mattis as defense secretary. Leaving the trump administration with two large holes in the global fight against terror. Tom.

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