President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Monday criticized outgoing Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, claiming he doesn't see countries that "take total advantage of the U.S." as "a problem."

But "I DO, and it is being fixed," Trump wrote in a tweet.

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Trump wrote in a series of tweets that he appreciates U.S. alliances but claimed that "many of these same countries take advantage of their friendship with the United States" and that the U.S. subsidizes the militaries of those countries.

"To those few Senators who think I don’t like or appreciate being allied with other countries, they are wrong, I DO. What I don’t like, however, is when many of these same countries take advantage of their friendship with the United States, both in Military Protection and Trade," Trump wrote in a tweet.

"We are substantially subsidizing the Militaries of many VERY rich countries all over the world, while at the same time these countries take total advantage of the U.S., and our TAXPAYERS, on Trade. General Mattis did not see this as a problem. I DO, and it is being fixed!" he continued.

To those few Senators who think I don’t like or appreciate being allied with other countries, they are wrong, I DO. What I don’t like, however, is when many of these same countries take advantage of their friendship with the United States, both in Military Protection and Trade... — Donald J. Trump (@realDonaldTrump) December 24, 2018

....We are substantially subsidizing the Militaries of many VERY rich countries all over the world, while at the same time these countries take total advantage of the U.S., and our TAXPAYERS, on Trade. General Mattis did not see this as a problem. I DO, and it is being fixed! — Donald J. Trump (@realDonaldTrump) December 24, 2018

Mattis last week resigned from his position as Defense secretary, prompted by Trump's decision to withdraw U.S. troops from Syria.

Mattis wrote in his resignation letter that the strength of the U.S. is "inextricably linked to the strength of our unique and comprehensive system of alliances." He added that the U.S. "cannot protect our interests" without "maintaining strong alliances and showing respect to those allies."

Mattis was scheduled to leave his position in February, but Trump announced Sunday that Mattis would instead leave on Jan. 1.

A senior administration official confirmed to The Hill on Sunday that Trump's anger over Mattis's resignation letter pushed the president to force him out early.