I think new presidents have no idea what they're getting into when they win. POTUS is contractually obliged to honor pre-existing defense contracts, security agreements, and the like. I think when they get into office they realize there is far less autonomy than they imagined.

Even in non-government business structures, the CEO is just a figurehead for the most part, and the board holds all of the power. In the case of the United States Corporation, the board of directors is the Federal Reserve. It's clear with the federal exchange rates of Obama's eight years in comparison to Trump's two years that the fed is punitively acting in accordance with his style of governance.

Question is: did he go into this wanting to be the policeman of the world, or did he resign himself to the racket that already existed via previous POTUS contracts? What would it take for a new president to violate existing security contracts, or what would the cost to the country be if he did?

They always create pretexts for wars because most Americans would not buy into the idea that the United States is the world's most infamous mercenary organization. Yet, the secret security agreements are bound to have clauses of non-disclosure in order to protect it from public scrutiny.