How could we prevent a worldwide peace-enforcing military from degenerating into a worldwide military dictatorship?
Maybe one way is to spread out the power in a hierarchy of checks and balances.
For example, each major region, such as Africa or South Asia, could have a peace-enforcing military responsible for that region. Sub-regions could also have their military forces. At a higher level, a world-wide force could either reinforce a regional military, or prevent it from turning into a force for repression.