His decision to leave the CPC in 1971 was primarily based upon his experiences within the Party from 1956 (the year of the Hungarian Revolution) up to, and after, the Soviet invasion of Prague in 1968. During his 35-year tenure in the CPC, Ryerson was routinely asked to augment his historical writings in order to meet the prevailing philosophy at the time. After the internal party crisis between 1956 and 1957, Ryerson was forced to write an article stating his previous books and articles had given "a rather idealised treatment of the bourgeois democrats Lafontaine and Baldwin." Blaming this on "liberalism," he essentially turned his back on his earlier beliefs concerning "1837" and sought to align himself with the new "revisionist" tendencies within the CPC that came about during the post-Stalin debate. Ryerson's beliefs concerning Marxism-Leninism differed greatly from that of the CPC of the late 1950s and early 1960s. His vision was brought to the forefront when in his article "In France: "The Week of Marxist Thought"," he agreed with the leader of the French Communist party who argued: