> What prevents this centralized body from making politically motivated decisions?
An established institutional culture of apolitical service, reinforced by respect for those who uphold it and consequences for those who violate it. The kind of thing that the U.S. still expects of its military, judiciary and public service, and the kind of thing that is corroded by political leaders showing contempt for it.
The US works by checks and balances. You can't point to, for example, the US military and say "see, we have the expectation with the military so it will transfer." You have to say, "See, the world holds respect for <$Centralized_Power>'s military despite centralized power."
Finding the value of $Centralized_Power might take a while.
An established institutional culture of apolitical service, reinforced by respect for those who uphold it and consequences for those who violate it. The kind of thing that the U.S. still expects of its military, judiciary and public service, and the kind of thing that is corroded by political leaders showing contempt for it.