[In 1958] there began a spell of bad luck and vexing failures in Bronstein's career. The cause of this was largely the Botvinnik initiated limit on the representatives from one country (i.e. the USSR) in the Candidates tournaments: not more than five out of eight were allowed.
In the Interzonal tournament in Portoroz 1958 there were six qualifiers, but the four Soviet grandmasters were fighting for only three places (since Smyslov and Keres were already in the list of Candidates), and willingly or not they were forced to play keeping an eye on one another. Both [Bronstein] and Averbakh missed out by half a step.
He was to suffer an even crueller stroke of fate at the Interzonal in Amsterdam 1964, where five Soviet grandmasters were fighting for the same three places. Another to suffer was Stein, who fell victim to the 'Botvinnik rule' for the second consecutive time.
It was effectively on account of this unjust restriction that two such splendid players as Bronstein and Stein missed out on the battle for the World Championship. Their replacements in the Candidates events were not of equivalent strength and it is clear that, with their participation, things at the top of chess could have turned out differently.
Alas, few now remember these human dramas. But meanwhile they reflected the unnatural situation that existed in chess in the 1950s and 1960s, when many of the best grandmasters, occupying most of the places in the world's top 30, had no opportunity to participate fully in the World Championship qualifying cycle -- for the only reason that they were Soviets.
Some will retort: but with this limit, fighting spirit was strengthened and only the very best made it to the top -- that is by the laws of Darwinism, the strongest survived. But just think how much nervous energy it cost these world-class stars to battle among themselves for the right to squeeze through the eye of a needle. And how it must have been to recognize that you are stronger and have occupied a higher place, but it is another player who will go through. Were not these unhealed spiritual wounds one of the causes of the untimely death of Leonid Stein?
(That last sentence deserves special attention, but it takes me too far from the subject at hand; this is not the right place to address it.) In the same chapter (p.215) Kasparov linked the 'Botvinnik rule' with the right to a return match.
In 1956, soon after Smyslov's second victory in the Candidates tournament, an event occurred that was to have a strong influence on the entire modern history of chess: FIDE granted the World Champion the right to a return match. The decision was adopted together with the aforementioned 'Botvinnik rule', and also not without the participation of [Botvinnik] (I should remind you that his friend Ragozin was a FIDE Vice-President).
Although earlier, in the late 1940s, in his plan for the contesting of the World Championship, he had rejected the idea of the return match, since 'it's organization would disturb the periodicity of the system, and in the interests of chess this must not be allowed', and he had gained the right for a defeated champion to play a match-tournament with the champion and the challenger three years later (this FIDE rule operated in the 1951 and 1954 matches).
The inescapable conclusion is that Botvinnik was behind both the limitation on Soviet players and the return match because these rules gave him better chances to retain his World Championship title. While I have no particular argument with this conclusion, I still wonder to what extent the 'Botvinnik rule' was a consequence of other federations' fears of Soviet collusion, as described in Collusion and Consequences. Perhaps it wasn't really a 'Botvinnik rule', but rather Botvinnik seeing personal advantage in a rule favored by the other chess federations.
In 'Predecessors IV' Kasparov dismissed the idea that Soviet collusion was responsible for Fischer's mediocre showing at Curacao in 1962. 'At that moment Bobby was not yet ready to win such a tournament -- irrespective of whether or not there was such a pact. Averbakh: "If Fischer could have beaten the Soviet grandmasters, as was to happen later in the early 1970s, no amount of draws would have been able to stop him."' (p.300). In other words, to win against collusion, a player had to be significantly better than the Soviet opposition. Tightly contested tournaments would always favor the Soviet bloc.
FIDE implemented matches to eliminate collusion in the Candidate tournaments. Was the primary objective of the earlier 'Botvinnik rule' to eliminate collusion in Interzonals rather than to favor Botvinnik?