"When a Good Man Goes to War" is best known as the title of the seventh
episode of the sixth series of the British science fiction television show
Doctor Who. This episode originally aired on June 4, 2011. It was written by
Steven Moffat and directed by Peter Hoar. The episode's plot involves the
Doctor and his allies mounting an assault on an asteroid base called Demons
Run to rescue Amy Pond and her abducted child. The phrase "when a good man
goes to war" is also part of a poetic refrain that appears in the episode,
spoken by the character River Song, and goes: "Demons run when a good man goes
to war,
Night will fall and drown the sun,
When a good man goes to war.
Friendship dies and true love lies,
Night will fall, and the dark will rise,
When a good man goes to war.
Demons run, but count the cost;
The battle's won, but the child is lost." This episode had high viewership and
was critically acclaimed, noted for its impact on the show's storyline and
character development.
In addition, "When a Good Man Goes to War" has also been used as the title for a song by the band Vexillum from their album released in 2021.
Thus, contextually "when a good man goes to war" is primarily recognized as a significant Doctor Who episode title and iconic line from that episode aired on June 4, 2011. It also exists as a song title in recent music. Let me know if you want details on one of these specific references or anything else about this phrase.