US President Donald Trump vowed Monday that allies would no longer "take advantage" of the United States, taking a shot at Defense Secretary Jim Mattis who resigned over Syria.

"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 We are substantially subsidizing the Militaries of many VERY rich countries all overt 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!"  Trump tweeted.