Trump makes mistakes: his governance is an iterative optimization; that's where the staffing churn comes from. It tends to work well in spaces that have ill-defined needs -- as long as updates are regular. With Trump, updates are event-immediate AND at planned intervals. Mattis was a tough-sounding general, a reputed Doer; he did a few things, then he schmucked. Now he's been excised, and the excision process (evidently) smarted a bit.