In his Brief of 1863, Pius IX set out principles for the Archbishop of Munich to repress the Liberalism of German Catholic intellectuals meeting at a congress in that city. This was opposed by Newman, who was in favor of free speech in the Church.
In a letter, posted below, written to Mr. Ornsby shortly after the publication of the Munich Brief, Newman expressed his position.
This document is in The Life of John Henry Cardinal Newman by Wilfrid Ward, vol. II, pp. 49-50.