Sorry, I mean Peace as in "we're not beefing with the communists right now".
Part of the reason, maybe the biggest part, that communist nations struggled is because the world's largest world power, which held 50% of the global GDP post WWII, openly opposed them as much as possible. It wasn't just war, it was trading, economic attacks, propaganda. Capitalism seeped back into communist nations because they saw the US was rich, whether that was true or not, and wanted to be more like the US. This was orchestrated intentionally over decades, this was literally the US' #1 goal.
And, to be fair, the US was rich. By circumstance. Post-WWII Europe was destroyed, we held 50% of the world GDP basically by default. Then our richness eroded as communist nations developed extraordinarily quickly, and threatened the US hegemony. Then we imploded our richness from within by throwing away the concessions FDR bought. The average American got poorer and poorer, and the golden age became known as that. Nobody really noticed because technology advanced, so people were poorer, but it was hard to tell. By the 80s people knew. Then we got Reagan, and things only deteriorated further.
That's not to say that communism is good or would have been good. It would have just eliminated a lot of strife in the world and would basically align all developed nations on one singular ideology. Would that ideology then be overthrown? We don't know, but I highly doubt it. People in the US would still be living good lives, except for a very small pool of elite few who got their shit rocked. Everyone else would still enjoy the advances of technology, and they would be content. The USSR would have been much more successful with the US ideologically and economically backing them. It would also be very hard to have a war at that point: over what? All of our post-WWII wars are over communism.
I mean, think about it. The USSR was the second world power, and it was very close! Despite the largest economy and military in the world doing everything in their power to ruin them. Imagine if, instead, those two powers worked together. Imagine where the USSR would be.
Also, the USSR was more progressive than the US. I'm talking gay right, rights for minorities. We would've gotten the civil rights era decades earlier. Gay marriage way, way earlier.
Anyway, I'm not a communist, and I'm happy with how the US turned out for the most part. I just think about how different history would have been if just one president was changed. I do truly think that if FDR was never president, most if not all developed nations would be communist to this day. I think people really do not appreciate just how close the US got to being communist. It was really, really close. We were going down the exact path of every communist nation that followed. There's some sources from the early 20th century, it's interesting to read.