It a situation like this, it seems to me like it would be necessary for the the Communists to have friends in the army (perhaps among the mid-ranking officers like the colonels and captains who in other countries proved susceptible to Communist ideology while also having the educations to be really dangerous and effective revolutionaries). A well placed major or colonel who had seen the paperwork for the (alleged) anti-Communist action could also provide the Communists warning of what was coming and enabling them to counter-act the army plot.
It would also be fun to see a TL about a Communist-Islamist alliance launching a coup and subsequently having to resist a civil war against the army (and whatever external pressure the US brought to bare). It's something I've found fascinating in Central Asia and the Middle East is how close Islamists and Communists came at some times (obviously, very different types of Islamism to that which currently makes the headlines today). A synthesis of the two into a theistic "Islamic Socialism" in Indonesia could be quite fascinating.
fasquardon
I agree with this. IOTL, we know that the army and islamist could take on the communist because they did, while we don't really know how strong the communist actually were because it was so fast and so curb-stomp-y. That's why earlier I stated that it is possible for a civil war if the communist getting stronger but not strong enough to take on the others. So, for Indonesia to be a communist state it is essential for the communist to build up their power base to the point they could take on the others.
But I must disagree that the communist and islamist could become an ally. IOTL, they were natural enemy. Before 30th september even, clashes between communist and islamist were rampant, especially after communist's attempt at unilaterally imposing land reform law of 1960, where many landlords were from
santri background/islamist leaning, clashing with the communist's main base of power in the
abangan people. So, if the communist won, I believe that it is almost certain that there will be mass killing as OTL, with reverse role for the perpetrator and victim.
If Indonesia goes communist, Australia is pretty much guaranteed to follow through with developing a true nuclear deterrence. Given the work and policies the Australian government followed OTL, it's not that far fetched.
This would have a lot of effects on Australian national security. I'm guessing their military would be a lot stronger in general. I wonder what would happen with Papuan independence. Australia might end up in a counterinsurgency war there, butterflying a lot of their involvement in Vietnam.
Probably a stronger naval air arm (maybe F-14s), ASW capabilities, and submarines to keep them at bay in the sea and in the air, which would be the main theaters of potential conflict.
IOTL, Indonesia did had nuclear ambition before 30th september event and regime change. Nuclear and conventional arms race between Australia and Indonesia seems possible.
About Papuan independence. IOTL, Papua New Guinea gained independence from Australia in 1975. If Indonesia goes communist, We could possibly see them staying longer, or if not, housed large number of Australian troops there. About Indonesian New Guinea/West Papua, this is where things goes really interesting. IOTL, the American pushed the Dutch to the negotiation table with Indonesia in 1963 in the hope that Indonesia will not go communist, and let the sham "act of free choice" got acknowledged because regime change put Indonesia firmly in anti-communist camp. If Indonesia goes communist, we could se the US do everything they can to avoid West Papua fall into Indonesian hand.
At risk of sounding Eurocentric, this would have interesting knock-on effects on Australian politics and pop culture. After all, Australian troops would be quite literally be fighting on Australia's border against an Asian Communist foe. Unfortunately might end up with even more pseudo-Yellow Peril feeling amongst anti-Communist publications as in OTL.
I imagine pictures like in this video, endless masses of enthusiastic Indonesian communist, will be common in Australia's television with the narration of "communist threat right next door. only you can stop it!"