If you fly into a restricted or prohibited airspace you may be intercepted by a U.S Military, law enforcement or republic fighter aircraft!
You must know how to react if are intercepted. The FAA provides guidance on what you should do.
FAA In-Flight Intercept Procedures
In 2015, the FAA published this document covering intercept procedures. It is good practice to have this or similar available to you in your flight bag / EFB for such an occurrence.