A preemptive arrest is one in which a person is arrested prior to committing a crime. Preemptive arrests are sometimes viewed with suspicion as being contrary to the principles of a democracy. In the United States federal system, conspiracy

Conspiracy (crime)

In the criminal law, a conspiracy is an agreement between two or more persons to break the law at some time in the future, and, in some cases, with at least one overt act in furtherance of that agreement...

to commit a crime in itself constitutes a crime, so an arrest for it would not be preemptive.