Mattis and the military commanders (whom Trump calls "my generals") have become quite uppitty against their commander in chief lately. Remember the generals response to Charlottesville? That was a clear shot at Trump. I notice that in his announcement on this issue, Mattis said that the panel that he's forming that will make recommendations on this issue "will bring mature experience, most notably in combat and deployed operations, and seasoned judgment to this task." Sounds to me like a shot aimed directly at Trump's lack of such mature experience and sound judgment.