The poetry and argument of the Countess sections feel like the kind of wordplay and cleverness one expects from the bard.

There is literally a line lifted from one of the sonnets in the play, so he either reused it or it was stolen.

The events and elements of Edward III are heavily mirrored and referenced in the play Henry V. It certainly seems Shakespeare had Edward III on his mind while writing his later history.

The “flaws” and failures of Edward III seem no different from those of Henry VI, part 1. Why exclude Edward III while keeping Henry VI, part 1 firmly in the canon?

Edward III was excluded from the First Folio, while Henry VI, part 1 was included. The First Folio was ostensibly Shakespeare’s complete and official work, published after his death by his two colleagues, John Heminges and Henry Condell. However, although their contribution to solidifying Shakespeare’s canon is invaluable, they also clearly got some things wrong, excluding plays such as Pericles and Two Noble Kinsmen, as well as having weaker versions of some of the plays than were already published in quarto form. This topic is rich and interesting, and I don’t have time to unpack it in this post. However, it does explain why Henry VI, part 1 gets so much acceptance even though it is arguably as weak as Edward III.

The Romantics idealized the genius poet working in solitude, and often excluded plays that seem imbalanced. This imbalance of quality can be explained by the fact of collaboration: two or more poets splitting the scenes to complete a work in less time or for other reasons. As Romantic as the lone poet seems, the vision doesn’t fit reality. We know from diaries of the time that collaboration occurred, and stylometric evidence shows multiple hands in as many as a third of the plays from Shakespeare’s day. To believe otherwise is subjective nonsense and false idealism.

Powerhouse scholars such as Harold Bloom have dismissed the play, a serious blow against it. However, his criticisms of a lack of psychological depth are purely subjective and could be leveled as easily against Henry VI, part 1. So.

Current stylometric studies (especially since 2009, when n-gram computational analyses were vastly improved) have attributed the language of the play to William Shakespeare and Thomas Kyd.

Since the 1990s, Yale, New Cambridge, Riverside, Oxford, and Arden have all begun attributing the play at least partially to Shakespeare with an ever-growing degree of confidence.