"I think that you never can underestimate the power of American values" seems to me to mean that it is not possible to esteem the power of American values to be as low as it actually is.

so person X says "I estimate the power of American values to be about a one on a 1-10 scale,"

Person Y says "I estimate that power to be even lower."

And Gates says, "go as low as you like, gentlemen, you'll never estimate it to be lower than it actually is."

This is something I would expect from the former administration when not being filmed (it's how I suspect Cheney, for example, feels about the country).

But now this may have to be something to expect from the Obama administration too. Meet the new defense secretary of war, same as the old defense secretary of war . . .

Herring405