Trump has gone out of his way in recent years to undermine our democracy. He questioned the legitimacy of President Obama, which was a long running campaign tactic to ensure his primacy with the white underclass. Then he sent a clear signal to his supporters that he would honor the results of the 2016 election only if he won, in the process sending a clear signal to the citizen militias.
Suppose Trump is impeached in the House and convicted in the Senate. Would the man with the nuclear codes and command of the military honor that process? Why would he? He could declare martial law at any stage in the process and keep the Congress from meeting on the impeachment, insisting that martial law is a necessary security measure. He could claim the U.S. is subject to an unspecified security threat that is classified. The 25th amendment to the Constitution provides special powers to the cabinet and Vice President to declare the President unfit for office, but if the President challenges their decision, Congress makes the decision and the President could use his military command to stop them from meeting.
We really have no foolproof method to stop a president who thinks only of himself and his personal aggrandizement.