When Trump took office, he assumed that everyone he appointed to an executive branch job was on his team—that they would support whatever he did and said, do whatever he wanted, no questions asked, no backtalk or criticism, public or private. If it ever came to a choice between Trump and “doing the right thing”, the team members would choose him, just as his staff had at the Trump Organization.
He quickly discovered it did not always work that way. And so among his other strategies, he saw that he would have to purge all those whose unconditional loyalty was beyond question, and replace them with those who, for whatever reasons (incompetence, ideology, need for job or job security, etc.), would toe the line.
These replacement players would be a compliant, sycophantic part of the wall—the wall Trump has almost completed around his executive branch. In the Justice Department, for example, Sessions and Comey are gone, as Rosenstein soon will be. The same has happened elsewhere, time and again.
This Trump wall, unlike the one at the southern border, will work, at least for a while. Built with the solid powers of the presidency, the wall won’t be easily gotten around or broken through. With all his glaring deficiencies, Trump knows one thing: how to protect himself. This wall around the executive branch will do just that.