James Mattis basically told the troops that their president is failing them

I suppose that, if asked to explain himself, Mattis could say that he was addressing troubles and tensions in the country in general, quite apart from anything the president had said or provoked. But in the wake of Charlottesville, only a dolt would believe that explanation. The target was clearly Trump, and if you don’t accept that, there was this:

“The power of inspiration—we’ll get the power of inspiration back.”

Mattis has referred to these “two powers” in many public forums before. In this instance, he says that we still have the power of intimidation (“that’s you,” he told the troops), but we’ve lost the power of inspiration for the moment (“we’ll get [that] back”). The only way to interpret this is that Mattis was saying the president—who is usually the one who embodies this power through words, deeds, or preferably both—has through his words and deeds surrendered this power.