The UEG even throughout the Interplanetary War and the conflicts that preceded it wasn't the totally united entity that it became post-war and throughout the millennia. It was more like a United Nations that had its shit together. However, that doesn't exclude the fact that this slightly more interconnected body of nations wasn't dominated totally by the foremost national powers.

The warfighting capability of UEG that beared down on the Friedenists and Koslovics in these conflicts across the Sol system were likely substantially provided by such nations like America, Britain, France, the People's Republic of China, and Russia. Each of them would have likely had their own space-faring forces and the capability to sustain their own extraterrestrial settlements.

So, in the case that the UEG had not won the Interplanetary War, there's quite a bit that would certainly change, but much would be the same. Earth would still have the greatest economical, military and political capability, and their drive to use it would have only sparked more interest within her nations to marginalize their victors. Technology, however, might possibly jump forward with more substantially weaponized starships and perhaps slipspace drives being introduced sooner to gain that critical edge in battle. As such, you would see more interstellar endeavors being undertaken much sooner than had actually happen.

But as we saw with the Insurrection and the incidents leading up to it, I'm sure the ferocity of Earth's imperial and colonial ambition out into the galaxy would only incite more violence leading up to its first interactions with the Covenant.