“Compulsory process” is a legal term that means that a defendant can appeal to the state to force witnesses to appear in court. Use of such force, through a court order called a “subpoena,” is often necessary, because witnesses are often reluctant to testify, for a variety of reasons. Sometimes witnesses are afraid; more commonly, they are simply reluctant to miss a day or more of work. A subpoena usually overcomes their reluctance.
The idea of calling witnesses seems so basic to us today that it seems hard to believe that, for much of British history, those who were accused of treason or felonies were not permitted to call witnesses in their own defense. William Hawkins, in his 1721 treatise on English criminal law, explained that prior to 1695, only the defendants in misdemeanor cases (misdemeanors are minor crimes; felonies are major crimes) had a right to this process; but “in Capital cases he hath no right by the Common law to any process against his witnesses without a special order of the court.”
According to Blackstone’s 1769 Commentaries on the Law of England, it was “to the honor of Mary I” that this ancient custom of denying witnesses in the defense of serious crimes began to be reversed, but the process was slow. Blackstone explained that “the courts grew so heartily ashamed of a doctrine so unreasonable and oppressive, that a practice was gradually introduced of examining witnesses for the prisoner, but not upon oath: the consequence of which still was, that the jury gave less credit to the prisoner’s evidence, than to that produced by the crown.” At last, primarily through the efforts of the House of Commons, it was established that all defendants “shall have the same compulsive process to bring in his witnesses for him, as was usual to compel their appearance against him.”