The Lincoln quote in the first sentence made my eyebrows lift. I thought the historical context might help to explain what Lincoln was talking about—and why Mattis thought the quotation apropos in the present circumstances.
The context, apparently, was that Confederate negotiators were in Washington trying to negotiate an end to the war. Lincoln was instructing Grant not to let the existence of those negotiations affect his military plans.
I assume Mattis is telling the troops not to let Trump’s dealings with our current enemies cause our troops to drop their guard.