I'd say that an even larger reason for federal-level law enforcement is the mobility that modern means of transportation brought. When the US was founded with its vision of autonomous states, there was not much mobility between states. It is no accident that the boom in the use of US marshals, postal police, and the Secret Service came with the building of the railways, making for crimes that spanned across states, for instance, while the FBI appeared in the automobile age.

Once you get national-level police that inherently see themselves are superior to and unaccountable to state-level bodies, then it is no surprise that they might want to wield that power, and that might have happened even without those foreign wars the US was involved in.