“Shortly after the Leon Trotsky Archive at Harvard’s Houghton Library was opened in January 1980, Trotskyist historian Pierre Broué discovered letters between Leon Sedov and his father Trotsky that proved the existence of a bloc between Trotskyites and other opposition groups within the USSR. Sometime in the middle of 1932 Sedov informed his father as follows:
“[The bloc] is organized. In it have entered the Zinovievites, the Sten-Lominadze group and the Trotskyites (former ‘[capitulators]’). The group of Safar. Tarkhkan has not formally entered yet – they stand on too extreme a position; they will enter in a very short time. – The declaration of Z. and K. concerning their enormous mistake in ’27 was made during negotiations with our people concerning the bloc, immediately before the exile of Z and K.” 
About the same time American historian Arch Getty was discovering that Trotsky had secretly sent letters to at least Radek, Sokol’nikov, Preobrazhenskii, Kollontai, and Litvinov. The first three had been Trotskyites before publicly recanting their views. Getty did not find the letters – only the certified mail receipts for them. Getty realized this meant that the Trotsky Archive has been ‘purged.’ These letters had been removed. Other materials had undoubtedly been purged as well. 
The only reason to “purge” the archives would have been to remove materials that would have seemed incriminating – that would have negatively impacted Trotsky’s reputation.As an examination of the question of the letter to Radek shows, the letters that we know were removed proved, at the very least, that Trotsky lied during the 1930s by claiming he never maintained contact with oppositionists inside the USSR when, in reality, he was doing so, and by claiming that he would never agree to a secret bloc between his supporters and other oppositionist groups in act he had done precisely that.
Evidently Broué found the implications of this fact very disturbing. He never mentioned Getty’s discoveries of Trotsky’s letters to his supporters and others inside the USSR or the purging of the Trotsky archive, even though Broué cites the same Getty publications (an article and a book) in a very positive manner. 
Therefore it has been well established by scholars by the mid-1980s that a Trotskyite-Zinovievite bloc did in fact exist and that it was formed in 1932 and that Zinoviev and Kamenev were personally involved. Sedov also foresaw the entry into the group of Safarov, who in any case had a group of his own.
In an interview with the Dutch social-democratic newspaper Het Volk during the second half of January 1937, at the time of the Second Moscow Trial, Sedov stated, in a slip of the tongue, that “the Trotskyists” had been in contact with the defendents at the First Moscow Trial of August 1936.  Sedov specifically named Zinoviev, Kamenev and Smirnov. Concerning Radek and Piatakov Sedov went on to say that “[t]he Trotskyists have had much less contact with them than with the others. To be exact: no contact at all.” That is, Sedov tried to withdraw his “slip” about Radek and Piatakov.
But Sedov did not even try and retract the information that preceded it: that “the Trotskyists” had indeed been in contact with “the other”: Smirnov, Zinoviev, and Kamenev. This interview, “slip of the tongue” included, was published in a provincial edition of Het Volk on January 28, 1937. It was noticed by the Communist press, which called attention to Sedov’s “slip of the tongue.” (Arbeideren, Oslo, February 5, 1937; Abejderbladet, Copenhagen, February 12, 1937.) Thanks to Getty we now know that the Communist press was correct. Sedov’s remark really was a “slip of the tongue.” We know that Sedov was lying because Getty had found evidence of Trotsky’s letter to Radek. Trotsky has indeed been in touch with Radek. Sedov’s first remark, about “much less contact,” was accurate.
Therefore we have good, non-Soviet evidence, confirmed by the Trotsky Archive, of the following:
- A “bloc” of Zinovievites, Trotskyites, and others including at least the Sten-Lominadze and, perhaps, the Safarov-Tarkhanov group (with whom they were in any case in touch) and involving Zinoviev and Kamenev themselves, was indeed formed in 1932.
- Trotsky had indeed been in touch with Zinoviev and Kamenev, as well as others, probably through his son and chief representative Sedov.
- Trotsky was indeed in touch with at least Radek and Piatakov.
- Trotsky really did send a letter to Radek, who was in Geneva at the time, in the Spring of 1932, just as Radek testified in the January 1932 Moscow Trial.
- There is no reason to accept Trotskyist historian Pierre Broué’s conclusion that thus bloc was “ephemeral” and died out shortly after it was formed, because we know the Trotsky Archive was purged at some time, while Broué had no evidence to support that statement.
The Harvard Trotsky archive yielded to Broué and Getty unmistakable evidence that the “bloc” did exist; that Trotsky was in contact with the bloc’s members and his own supporters inside the USSR, and that Trotsky lied consistently about all these matters both in the Bulletin of the Opposition and to the Dewey Commission. No scholar today denies this. Kirilina, Lenoe, and Egge simply ignore the whole matter.
The NKVD of the 1930s termed the complexly-interlocking set of oppositional conspiracies the “klubok,” or “tangle.” If any of these conspiracies were acknowledged to have existed, it would be hard to deny the existence of the rest, since all the defendants implicated others in a chant that, directly or indirectly, connected them all. Admitting that the bloc of Trotskyites and Zinovievites did in fact exist would present the danger of a “slippery slope” to any historian who wanted to deny the validity of the other conspiracies. For once it is conceded that the first alleged underground opposition conspiracy really did exist, and therefore that both the Khrushchev and Gorbachev official reports, rehabilitations, and official historians were lying, it logically follows that other conspiracies, which these same sources also denied, might have existed too.
– Grover Furr, “The Murder of Sergei Kirov: History, Scholarship and the Anti-Stalin Paradigm,” pages 131-133, 136.
 Harvard, Trotsky Archive 4782 p. 1; see Broué in Cahiers Leon Trotsky 1980 p. 36; Broué, “Party Opposition to Stalin…” p. 100.
 Other materials were certainly removed – “purged” – with traces of their removal remaining. For example, excerpts from a discussion between Trotsky and Sedov concerning the slogan “remove Stalin” (‘ubrat Stalina’) remain in the archive, but the full letters from which the excerpts were made are not there.
 We will examine this whole question in detail in a forthcoming work.
 “Het process te Moskou. Wie niet wil bekennen al doodgeschoten?” Het Volk 28 Jan. 1937 pp. 1 and 5. My sincere thanks to Sven-Eric Holmstrom for tracking down this article and generously providing me with a copy.