The Communist victory against overwhelming numbers and overwhelming odds always struck me as a very improbable occurence. But was its occurence really such a "fragile" thing that depended on many circumstances to be tuned exactly in its favour? Or was it a certainty that was largely impervious to changes of circumstance? And if so, beyond what point was it so certain?
Prior to WWII it wasn't that probable. The Warlord Period of China was an abject mess and even the KMT didn't have full control of the country. The Long March was necessary precisely because Chiang was in the process of annihilating the proto-PLA forces in 1934.
On several occasions Mao "thanked" his major enemies (Chiang, Japan, and the US) essentially for providing trials to unify the Communist cause. It's easy to find certain quotes where the thanks Japan for weakening the Nationalists to the point where the Communists could win but many of them seem to be made up and without real sourcing. However, it's hard to argue that the Communists didn't indeed benefit from Imperial Japan's conflict with the KMT given that the KMT did a great deal more of the actual fighting.