I’m a Marine veteran. A recently retired Marine shouldn’t be defense secretary.

As a combat veteran of the Marine Corps and a longtime admirer of Gen. James N. Mattis, I was sad to annouce this month that I could not in good conscience support granting him a waiver to serve as our 26th secretary of defense.

I explained that my decision was motivated not by political considerations but by concern for the enduring American principle of civilian control of the military. I lauded Mattis’s eminent qualifications and leadership skills while affirming that this central tenent of our democracy should matter more than any single individual.

What happened next was revealing.

Despite my considered words, I got an earful from Marines across the country, including men I served alongside in Iraq. They called and wrote letters. They tweeted and texted. In some colorful language that I can’t repeat in this space, they questioned my loyalty to the Marine Corps and to our country.