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Amidst their continuous expansion and their technological growth, both the Republic and the League allowed themselves plenty of time for war.
The same year that Second Contact was made, the First Interstellar War was declared, to the surprise of no one except the NorAellians, who had believed that they had stabilized human relations by giving the Republic what they had given the League.
The Terrans based their war declaration on two things:
Firstly, the absolute destruction of the First Fleet over Mars through the use of nuclear weapons. Republic officials referred to it as a trap for a mission meant to re-establish diplomatic ties between Mars and Earth, which historians would later point out is technically true but falls far short of truly describing what that fleet was intended to do.
Secondly, the Republic claimed, correctly, that the League intended to utilize their higher technology level to crush the Republic and conquer Earth themselves and that it was only through the gracious intervention of the NorAellians that this villainous scheme failed. League representatives have never refuted the claim, and though they have protested the use of the word "conquer", they haven't ever been able to provide a viable explanation of how what they would have needed to do to remove the Republic government could have been anything but conquest.
This first war waged amongst the stars lasted 27 years and consisted of little more than small border skirmishes. Neither government had the full fleets that would later characterize the wars between humanity, and as a result, the First Interstellar War was, up until its end, a war that did not see massive numbers of casualties. This was also aided by neither side truly knowing how an interstellar war was waged, though humans are nothing if not amazing at learning new skills.
In 2288, a Terran Strike Team penetrated League space all the way to the Calysto System and, in a move meant as revenge for the the loss of the Terran First Fleet, detonated a nuclear device inside the League's primary shipyard at the time. An entire League fleet was being constructed at the time, consisting of the newest and most advanced ships that humanity had constructed to date and the blast destroyed or irradiated all of them, setting League defensive development back a decade.
This strike had three major effects: it placed the League and the Republic on more or less equal footing in terms of military armament, it brought the League to the negotiating table in early 2289, and it saw both sides agree that nuclear strikes should be restricted to ship-to-ship combat only for all future conflicts.
All in all, historians have largely decided that the Republic was more or less the "winner" of this first war, as they declared it and their actions ended it, however, neither side landed a truly devastating blow on the other outside of the Republic's strike on the Calysto Yards, and neither side was able to conquer any systems of true importance, instead they just swapped border worlds back and forth. The trading of border colonies would go on to be such a hallmark of these wars that by the time the Confederacy War was concluded, capturing a border colony was seen as little more than a nuisance and failed to even show up as a Causus Belli in future wars.
This first cease-fire agreement lasted an impressive 61 years, the longest single span of time that peace has existed between the Terrans and the League since both became interstellar powers.