It doesn’t say “I will interpret the constitution and decide if it’s being followed and if not stage a coup,” though. Interpreting is the judiciary’s job. The military is at the beck and call of the executive, who is checked and balanced by the judiciary and legislative. If the executive branch wants to implement socialism, it’s up to the other two branches of government to stop him, and if he fails to listen to them, they order the military to carry out their previously determined impeachment, and remove him from office. Thus, the military cannot intervene unless directed to in a manner consistent with the constitution.
Thanks for knocking down that straw man. The oath doesn’t say to intrepret the constitution. It says to support and defend the constitution against all enemies, foreign and domestic. No interpreters, no philosophers needed. When someone attacks you, you don’t need a philosopher to tell you someone is attacking you.