The symphony is widely regarded as a mature expression of the classical style of the late eighteenth century that also exhibits defining features of the romantic style that would hold sway in the nineteenth century. The Third was begun immediately after the Second, completed in August 1804, and first performed 7 April 1805.
The work is a milestone in the history of the classical symphony for a number of reasons. The piece is about twice as long as symphonies by Haydn or Mozart—the first movement alone is almost as long as many Classical symphonies, if the exposition repeat is observed. The work covers more emotional ground than earlier works had, and is often cited as the beginning of the Romantic period in music. The second movement, in particular, displays a great range of emotion, from the misery of the main funeral march theme, to the relative solace of happier, major key episodes. The finale of the symphony shows a similar range, and is given an importance in the overall scheme which was virtually unheard of previously — whereas in earlier symphonies, the finale was a quick and breezy finishing off, here it is a lengthy set of variations and fugue on a theme Beethoven had originally written for his ballet music The Creatures of Prometheus.
The opening of Eroica is similar to Mozart's overture in Bastien und Bastienne. It is doubtful that Beethoven was familiar with this unpublished piece. A likely explanation is that both composers took the theme from another unknown source. Robert W. Gutman believes that the similarity is coincidental. The late works of Mozart show steps towards the romanticism more fully embraced by Beethoven, highlighting that the Eroica did not arrive fully sui generis.