Will there ever be a Eurasian nation?
Why is Oceania only waging war against another nation?
It's probably just a coincidence.
According to Emmanuel Goldstein's book, there is in the book in 1984 1984 only three political entities on earth. I'll call them "countries" here, but they're more like empires or conglomerates. I'm not sure we will ever be given the explicit term for them, even in Newspeak.
These 3 countries are always at war with each other. The two times we're told which side is which, Oceania, Winston's homeland, is first allied with Eurasia against East Asia, then it switches sides to be allied with East Asia against Eurasia (maybe these two will switch, I have haven't read the book in a while).
We don't know if Oceania will ever align itself against both countries at the same time, but it could be. The war has been going on for 25 years, according to Emmanuel, and it will likely last forever, or at least a very, very long time, and with the party rewriting history all the time, not many people would remember it years later, even if it did happen and had almost no way of knowing when alliances changed before they were born.
My point is, we only ever see two examples of the alliances in this war, and the book probably only lasts a few months in what, according to the most reliable source we have outside of Winston (which doesn't say much), It was already a 25-year war that will never end. At no other time does Winston say for sure when the alliances shifted, likely because he doesn't remember and can't look up the information - even if he could, he knows it wouldn't be correct. You seem to be trying to extrapolate from two examples to a whole endless war.
If you want to see the relevant quotes, @Richard's answer has a few.
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