It is a Latin expression used in common speech and in the legal system. It means in fact. In your example, the police are not usually tasked with enforcing immigration laws. In fact, they are obliged to enforce immigration laws. The group has no designated leader. Peter is the de facto leader. That means that Peter is not formally designated the leader of the groups but in reality, he performs that role.

"De facto" describes a rule, policy or situation that exists in reality though it was never formally written into law. It stands in contrast to "du jure" which describes something that has been officially written into law.

passed state legislatures = means that these laws went through the part of the government concerned with law making successfully. act under = here, it means the police had to act because of a law. federal mandate = a regulation passed by the US Congress de facto = existing or holding a specified position in fact but not necessarily by legal right So the police were almost forced to di the job immigration agents because of a ruling by Congress.

* correction* de jure