The conflict between Brahms and his allies and the proponents of the New German School resulted in a “manifesto” written by Brahms and published in the Berliner Musik-Zeitung Echo in 1860:
The undersigned have long followed with regret the proceedings of a certain party whose organ is Brendel’s Zeitschrift für Musik. The said Zeitschrift unceasingly promulgates the theory that the most prominent striving musicians are in accord with the aims represented in its pages, that they recognise in the compositions of the leaders of the new school works of artistic value, and that the contention for and against the so-called Music of the Future has been finally fought out, especially in North Germany, and decided in its favour. The undersigned regard it as their duty to protest against such a distortion of fact, and declare, at least for their own part, that they do not acknowledge the principles avowed by the Zeitschrift, and that they can only lament and condemn the productions of the leaders and pupils of the so-called New-German school, which on the one hand apply those principles practically, and on the other necessitate the constant setting up of new and unheard-of theories which are contrary to the very nature of music.
A few days later, an answer appeared in the Neue Zeitschrift:
Dread Mr. Editor,
All is out!——I learn that a political coup has been carried out, the entire new world rooted out stump and branch, and Weimar and Leipzig, especially, struck out of the musical map of the world. To compass this end, a widely outreaching letter was thought out and sent out to the chosen-out faithful of all lands, in which strongly outspoken protest was made against the increasing epidemic of the Music of the Future. Amongst the select of the out-worthies [paragons] are to be reckoned several outsiders whose names, however, the modern historian of art has not been able to find out. Nevertheless, should the avalanche of signatures widen out sufficiently, the storm will break out suddenly. Although the strictest secrecy has been enjoined upon the chosen-out by the hatchers-out of this musico-tragic out-and-outer, I have succeeded in obtaining sight of the original, and I am glad, dread Mr. Editor, to be able to communicate to you, in what follows, the contents of this aptly conceived state paper—I remain, yours most truly,
Office of the Music of the Future [Zukunftsmusik]
Brahms despised Liszt’s music, and was widely believed to hold the same low opinion of Wagner’s. Brahms and Wagner were each competing, as it were, to wear the mantle of Beethoven and carry the genius of Germanic music into a new era. However, Brahms quite clearly paid homage to Wagner in the second movement of his Symphony no. 1 in C minor, op. 68.
The symphony’s second movement contains several obvious allusions to Wagner’s groundbreaking “Tristan chord” (movement 2 starts at 12:52):
The Tristan chord occurs first in the prelude of Wagner’s 1865 opera Tristan und Isolde, and consists of F-B-D#-G#: an augmented fourth, sixth, and ninth. Any chord that contained these intervallic relationships became known as a Tristan chord.
More on the Tristan chord:
Brahms was a collector of manuscript scores, and had an autograph score of a scene from Wagner’s opera Tannhäuser. When Wagner found out, he demanded that Brahms return it to him. They exchanged frosty letters, which you can read here, and Brahms eventually did return the score. Wagner relented by sending him a first-edition of Das Rheingold.