From early 1869, before the dogma of Papal Infallibility was promulgated (July 18, 1870), the Letters of Janus by Ignaz von Döllinger, the main leader of German Catholic Liberalism, were regularly published in England. According to him, to proclaim such a dogma was wrong or, at the least, very inopportune. His theses were quite similar to those held by Newman in his letters.
During the First Vatican Council, a minority of Bishops also strongly opposed that dogma. As a manifestation of protest, they refused to be present at its solemn promulgation, leaving Rome some days before.
In that tense situation, when it was uncertain what the Liberal Catholics would do, Newman seemed to be preparing to join them in public resistance against the dogma. The two letters we reproduce below bear witness to this probability.
Later on, in March 1871, after clamorous opposition to the dogma and an explicit refusal to sign it, Döllinger was excommunicated and became a heretic - an Old Catholic. The dissident Bishops, however, were addressed one by one by the Holy See and consequently accepted the dogma. Liberal Catholicism thus suffered a considerable defeat. As a result, Newman also gave up his opposition.
The two documents below, dated August 7 and 8, 1870, were written while the climate of opposition to the dogma was reaching its apex in Germany and France. In parallel, it is interesting to see how Newman, after the proclamation of the dogma, pretended that he had always held the same doctrine on that dogma that he had violently criticized before. An opportunist politician, we would say, rather then a model of virtue...
Below, photocopy n. 1 displays a letter (August 7, 1870) to Mr. O'Neill Daunt, advising him how to deal with a friend against Papal Infallibility. In it is clear that Newman repeats the liberal arguments against the dogma and pretends that those who do not accept it will continue to be "good Catholics." In photocopy n. 2 is a letter (August 8, 1870) to one of his regular correspondents, Mrs. Froude, in which he confesses that he is accepting the dogma for the safety's sake while he waits to see what the Liberal Bishops will do.
These document are in The Life of John Henry Cardinal Newman by Wilfrid Ward, vol. II, pp. 310-311 and 308-309 respectively.