I am always surprised about the Cambridge Five and the theft of nuclear secrets from the U.S. by the Soviets. They created their union in 1922 but had infiltrated the West enough to plant operatives at the highest levels, in numerous countries, all in about 25 years.
Did it have something to do with ideology? The doctrine of socialism may have attracted many people to spying for the USSR.
The early "moles" were all "ideological" spies — they were people who sympathized with the USSR because of its ideology. They believed, for example, that Stalin was creating a new, better society, and that he was going to need all the help he could get beating the Germans and, ultimately, balancing power against the "imperialists" like the United Kingdom and United States.
They generally did not "plant operatives" so much as recruit "volunteers" and sometimes steer them in places they'd like them to go. It was not a James Bond sort of thing, it was a John LeCarré sort of thing. The USSR did have trained intelligence agents (the James Bond types), but their role here was mostly to coordinate and collect the information, not to do the actual "spying" itself. (This is a generalization, but holds for the Cambridge five and most of the atomic spies. A few spies don't fit this mold, like George Koval, who actually was trained in the USSR, but I wouldn't call him James Bond.)
It is hard to imagine today but there were many intelligent people in the 1930s and 1940s who thought Stalin was great, that any suggestions to the contrary were exaggerations or lies, and so forth.
Later "moles" were motivated by things other than ideology, e.g. money, coercion, ego (the "MICE" rubric for spy motivations, when combined with ideology).