"This Day We Fight!" is a song by the American heavy metal band Megadeth, which appears on their twelfth studio album Endgame, which was released on September 15, 2009, written by frontman Dave Mustaine. It is the second song on the album, and has been played live together with album's first track, "Dialectic Chaos", which has brought positive comparisons to the two opening tracks "Into the Lungs of Hell" and "Set the World Afire" from Megadeth's 1988 album, So Far, So Good... So What!.

The song was based on the mythos of Tolkien's The Lord of the Rings, and was featured as a playable song on Guitar Hero: Warriors of Rock, along with both "Holy Wars... The Punishment Due" and "Sudden Death".

The song, like a vast majority of Megadeth's songs, was written by Dave Mustaine, and was lyrically inspired by J. R. R. Tolkien's The Lord of the Rings trilogy, particularly Aragorn's speech at the Black Gate of Mordor during the Battle of the Morannon, as well as Théoden's speech at the Battle of the Pelennor Fields. "This Day We Fight" is actually a direct quote from the film version of The Return of the King. The song was also further inspired by "warrior creed from the great Sun Tzu with the use of drums and flags in ancient war". Another song from the album, "How The Story Ends", drew inspiration from the same sources, with both songs containing what was considered a "brutal, blood-thirsty and aggressive" aesthetic which drew inspiration from violent fantasy films.

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