Why would any state help another state acquire nuclear weapons? This is a general question (it has happened many times — the USSR and the PRC, France and Israel, China and Pakistan, Pakistan and lots of people, etc.), and has many different possible answers. The political scientist Matthew Kroenig has a theory (in his Exporting the Bomb) that I find plausible in many cases (but not all), which is that typically when one state is willing to help another state become a regional nuclear power, it is a reflection of the fact that the "supplier" state is not worried about the bomb complicating their own hegemony in that region (because they may not have any stake there), but it will complicate the hegemony of another state. So France helped Israel get the bomb fairly explicitly because they thought it would complicate the situation in the Arab world, which would help the French get leverage in Algeria. China helped Pakistan get the bomb because it complicated things with India, which served China's aims. Under this theory, the USSR helped China because it complicated things in Asia with regards to the United States — which it certainly did.

But wait, wouldn't it complicate things with the USSR in that region? Kroenig's argument is that in the period that the USSR was willing to help China become a big player in East Asia, the USSR had more or less given up on its imperial ambitions over there. If the USSR had had more global ambitions, it would have been much more wary.

(This theory gives a good, easy explanation of why the USA was extremely unwilling to help any nation acquire the bomb — it always had global, hegemonic ambitions. It did assist some nations with furthering their arsenals once they had "proven" they already knew how to make them, though.)

The Chinese started to push for nuclear weapons in the mid-1950s, during the first Taiwan Strait Crisis, after President Eisenhower explicitly used the tactic of "nuclear blackmail" against them — if they invaded Taiwan, he might use nuclear weapons as a response (even though the Chinese were not nuclear armed). Mao realized that if China lacked nuclear weapons the USA would continue to do this sort of thing indefinitely.

The Chinese had no confidence in the Soviet nuclear umbrella. They were right not to — it wasn't clear the Soviets would actually go to war over them, and the Soviet nuclear umbrella was much more paltry than the American one until the 1970s. China, incidentally, was targeted by early US nuclear war plans, whether they were part of whatever conflict the sparked it or not (there was not a "don't nuke China" option in the early plans), so they certainly would have suffered the negative side of being under the umbrella.

What's kind of amazing about the Soviet assistance to China is that it started rather late — not until 1955 did they formally begin helping the Chinese. They did this in spite of many leaders in the Soviet Politburo, notably Khrushchev, thinking that there were going to be inevitable conflicts with the Chinese over matters of doctrine and policy (e.g. Khrushchev was already planning to break with the Stalinist past; Mao continued to embrace Stalin).

What advantages, at this stage, would it be for the Soviets to give assistance? OK, there is the ostensible one of helping an "ally" against a common foe — necessary but perhaps not sufficient. Part of the motivation may have been to keep the Chinese somewhat dependent. If the Soviets were supplying the technological know-how, if they had the "keys to the factories" so to speak, then the Chinese would be reliant on them. And indeed, the Soviets did not give the Chinese total control over the facilities, nor explain to them exactly how they worked, nor give them a "educational bomb" they had once promised them. When the Soviets pulled out in 1959 — a mere four years later! — they left the Chinese in a complicated, compromised position. The Chinese knew many things about uranium enrichment, but they didn't know how to make the whole process work. It took considerable effort (another 5 years) on behalf of the Chinese to "re-learn" all of these things and get a bomb developed. At first the Chinese technicians simply tried (under political pressure) to run Soviet equipment without understanding it deeply, and caused hundreds of accidents in the process. Later they endeavored to completely understand the technology first, and had much more success, but their path to the bomb took 10 full years to complete, no quick acquisition. (On the Chinese side of things, there is still no better source than John Wilson Lewis and Xue Litai, China Builds the Bomb, 1988.)

We are preconditioned to see "atomic assistance" as furthering a nuclear program; it may do the exact opposite. Political scientist Alex Montgomery has argued in a wonderfully-titled paper, "Stop Helping Me", that in fact, most instances of foreign nuclear assistance slows indigenous nuclear programs. This is counter-intuitive unless you study technological transfer and organizations deeply, but the essential argument is one that any student has experienced: if you are simply given an answer, you may have solved a single problem easily, but you are far less ready to tackle new problems than if you had struggled through the process of education yourself. Similarly, if you go to a country, give them a complicated factory they do not know how to use, staff it with your own employees (who do not tell them how it works), then withdraw all of the staff (and some of the key pieces of equipment that are portable), you will have actually dragged out arguably longer the time it will take for them to get it working, as opposed to if they had, from the beginning, invested their time and labor into learning how to develop it all indigenously.

While I don't think the Soviets thought that their help would actively slow the Chinese bomb program, I do think they concluded that it was better for them to be involved with it, and to have the Chinese be dependent on them. If the Soviets were involved, they could monitor its progress, and (as they did) potentially pull out if they weren't happy with the Chinese. It could be a bargaining chip, in other words, as well as an excuse to see what the Chinese were doing on this front. (The US did similar things with non-military nuclear programs in European states.)

I understood relations soured after the 1955 Sino-Sov split?