Presidents have always sent people to lead the Pentagon who respect the institutions and personnel of the armed forces, not least because Americans tend to bristle at any sign that an administration does not unreservedly support the men and women of the U.S. military. (Just ask Bill Clinton and Barack Obama, both of whom were castigated for such supposed disrespect.) In his first term, Donald Trump sent General James Mattis, a veteran of wars in Iraq and Afghanistan, and then, when Mattis quit, he appointed a long-serving defense professional, Mark Esper. But this time, the president found a perfect instrument...