WASHINGTON – Whether it's mocking "haters" or attacking retired four-star general Stanley McChrystal for criticizing him, President Donald Trump served New Year's Day notice Tuesday that he is not soothing his aggressive tone.

"'General” McChrystal got fired like a dog by Obama," Trump tweeted on the first day of the new year. "Last assignment a total bust. Known for big, dumb mouth. Hillary lover!"

The tweet came two days after McChrystal described the president as dishonest and immoral.

Earlier in the day, Trump turned a holiday expression into a mocking attack on his critics, urging them to "calm down and enjoy the ride" as the nation heads into 2019.

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Wrote Trump: "HAPPY NEW YEAR TO EVERYONE, INCLUDING THE HATERS AND THE FAKE NEWS MEDIA! 2019 WILL BE A FANTASTIC YEAR FOR THOSE NOT SUFFERING FROM TRUMP DERANGEMENT SYNDROME. JUST CALM DOWN AND ENJOY THE RIDE, GREAT THINGS ARE HAPPENING FOR OUR COUNTRY!"

McChrystal, the commander of U.S. forces in Afghanistan, was fired in 2010 by President Barack Obama over critical comments about the administration.

On a Sunday broadcast, McChrystal told ABC News' This Week that he would never work for Trump.

"I think it's important for me to work for people who I think are basically honest, who tell the truth as best they know it," McChrystal told ABC's Martha Raddatz.

“You think he’s a liar?” Raddatz asked.

"I don't think he tells the truth," McChrystal replied.

"Is Trump immoral, in your view?" Raddatz then asked.

"I think he is," McChrystal said.