: Well, as far as I'm aware, the UEG only took shape AFTER the Interplanetary
: War was won by the UN. So that means the war was fought by the UN and its
: UNSC. I haven't read the literature in a long time,though, so I'm not
: sure.

The UEG was established in the late 21st century but only became the wholly unified entity we see in the 26th century following the war. The United Nations at that point was pretty much synonymous with the UEG as the off-world colonies it did possess would have been very small and entirely dependent on Earth.

: That is definitely correct. What I'm not sure about is China and Russia,
: though.

I'd imagine that those two are as peachy as ever.

: The Koslovics were communist, so that would mean that Communist China either
: fought against them as part of the UN, stayed neutral due to their
: communist sympathies, or left the UN outright and supported the Koslovics.
: Though the latter would have added the additional threat of a China-UN
: ground war on Earth, with possibly nuclear implications, so I don't think
: that's plausible.

I think all options are unlikely as China, in whatever form it will become, has remained one of Earth's highest grossing industrial powers since the early colonial-era. It would definitely be an integral component of an interconnected Earth system as a part of the UEG.

: As for Russia, I have some doubts that they'd support a US-led UNSC, but I
: don't think they'd have any choice.

Mind you that it is the United Nations Space Command and while much of its military organization are derived from the American and Commonwealth system, Russia would definitely have a significant presence in its ranks. The Soyuz platform today remains the only delivery system for personnel to the International Space Station and its cosmonauts are routinely part of expedition crews. Russia will always have a significant role to play in space exploration and that will surely be the case when that includes military hardware.

: Or perhaps China and Russia in 2160 are no longer anti-US. Perhaps China
: isn't even Communist anymore. We can only speculate since we know
: literally almost nothing about this period.

China isn't so much anti-American as they are trying to have balance. They're trying to flex their muscles in an American-dominated Pacific and pissing off everyone but themselves in the process. This is making their neighbors have increasingly weary of Chinese expansion and influence, and finding themselves more in line as partners with the United States.

Russia? Ehh, that's just Russia being Russia. We can only hope that tensions lower as Putin and the last vestiges of those loyal wither away.