While there is little doubt that Shakespeare wrote King John, there has been much discussion concerning his inspiration for this history play. Scholars have debated over the sources available to Shakespeare at the time he composed the play and how far he departed from those sources with regard to his play's structure and characterization. Further, modern critics have speculated on the extent to which Elizabethan politics and current events might have influenced the playwright's treatment of such issues as succession, legitimacy, and loyalty to the crown.
The two sources most frequently cited for King John are the 1587 edition of Raphael Holinshed's Chronicles of England, Scotland, and Ireland, and an anonymous play entitled The Troublesome Reign of King John which is believed to have been published in the early 1590s. Geoffrey Bullough (1962) notes that prior to the twentieth century, some critics asserted that The Troublesome Reign was in fact authored by Shakespeare himself. Today, most scholars reject this theory and many share Bullough's view that Shakespeare relied upon the plot of The Troublesome Reign simply as source material—significantly rewriting the anonymous play by changing the emphasis and style to create his own King John. An alternative outlook is presented by Brian Boyd (1995). He argues that the inferior Troublesome Reign could not have inspired Shakespeare's King John but instead was written afterward and that its anonymous author used Shakespeare's play as a framework for his anti-Catholic sentiments.
Questions regarding the background material for King John have led scholars to examine the ways in which these sources have been modified. Accordingly, much attention has been devoted to Shakespeare's invention of Philip Faulconbridge, the Bastard. Alexander Leggati (1977) suggests that in his role as a dominant character, the Bastard reflects the play's changing perspectives from satire to patriotism, and from there to the practical requirements of politics.
The significance of the Bastard and of the play's volatile and dangerous world is the subject as well of James L. Calderwood's analysis (1960) of commodity or self-interest and honor, in which he points out that these two antagonistic but surprisingly similar themes give King John "a unity of structure generally denied it." Adrien Bonjour (1951) also finds structural unity through the character of the Bastard—in this case, through Faulconbridge's "closely connected" but "complementary" association with King John.
That most of the play's characters are governed by realpolitik rather than by idealism is emphasized by Virginia M. Vaughan (1989) and L. A. Beaurline (1990). Beaurline remarks on the brutal attitude of King John and on the opportunism of Pandulph concerning the fate of the child Arthur. Vaughan sees such attitudes as the play's acknowledgment of the changing times, or the "shift from a medieval ideal of communal responsibility to modern individualism. . . ."
Finally, several critics note the subversive elements in King John. Phyllis Rackin (1990), for instance, concedes that the roles of the female characters are brief, yet points out that while Constance and Elinor are onstage "they set the subversive keynote for the [male] characters" by continually threatening to disrupt their ambitions. Turning to Renaissance society, William H. Matchett (1962) observes that the play focuses in part on the right of succession, an issue that was a particularly sensitive one to the unmarried and childless Elizabeth I. Summing up, John R. Elliot (1965) asserts that "in King John, at least, Shakespeare chose deliberately to dramatize the most controversial material to be found in his sources."