READ IT: Defense Secretary Jim Mattis takes aim at Trump's Syria pullout in resignation letter

In his resignation letter, Defense Secretary Jim Mattis made jabs at Donald Trump's decision to withdraw from Syria -- and his apparent disagreement with it.

In the letter, the outgoing defense secretary mentioned the American military commitment to its allies -- a statement that echoes bipartisan critics of the president's Syria withdrawal who say it will alienate and endanger America's Kurdish allies.

"Our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships," Mattis wrote. "While the US remains the indispensable nation in the free world, we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies."

"I believe we must be resolute and unambiguous in our approach to those countries whose strategic interests are increasingly in tension with ours," he added. "It is clear that China and Russia, for example, want to shape a world consistent with their authoritarian model -- gaining veto authority over other nations' economic, diplomatic, and security decisions -- to promote their own itnerests at the expense of their neighbors, America and our allies. That is why we must use all the tools of American power to to provide for the common defense."

"My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues," Mattis noted.

"Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects," he wrote, "I believe it is right for me to step down from my position."

You can read the entire letter below: