The Yamatai Star Empire has a history of conflict with the Lorath Matriarchy.
After the discovery of Lor in YE 28 by Rufus Sydney of the YSS Sakura, the arrogance of Lor's diplomats displeased the Taisa and he shot diplomat “Code” in the head. Lorath leaders were strongarmed into signing a treaty, making them part of the Yamatai Star Empire.
During the First Mishhuvurthyar War, in YE 29, Lorath Queen Xiaah Harr'Ikke "Destiny" Tur'Lista met with Melisson on her Shlarvasseroth-class Flagship. After the discussion, Melisson moved one of Lor's moons into a collision course with Lor. Despite efforts by Hanako to stop the moon by obliterating it, the planet was heavily damaged on one side by falling meteor debris, leaving the Lorath with a lasting hatred of not only the Mishhuvurthyar, but for Yamatai for getting them involved.
During another visit to Lor, Hanako was seduced by the Lorath princess and Hanako took her virginity that had been reserved for High Priest Velor Tomoe Tur’lista. The resulting scandal and cover-up led to a civil war between the Occhestan class and other Lorath groups.
Lor left Yamatai along with the rest of the United Outer Colonies in YE 30. This greatly displeased Yamatai.
Yamatai briefly blockaded the Lor system in early YE 301).
The UOC, having been a splinter nation of Yamatai that is no longer capable of self-sufficiency, is re-instated in the Yamatai Star Empire in YE 33. There's no specific law stating this, but it's a given for the YSE to the point they don't think about it - they just act as if it is so. The technical reason the YSE gave was that the separation of the UOC was part of the treaty they made - and since the UOC gov went into hiding (Where the hell did Emperor Tio even go??) the treaty was void and the territorial agreement thereof was null, leaving the YSE to reclaim anything the UOC had. This reason was further fueled by the YSE public's desire to rescue the UOC after learning of their situation via Task Force Torch's investigation. Having the entirety of the UOC on paper as YSE citizens allowed the Star Army to go in and remove them legally, for their own protection. Accordingly, the Star Army did go in and save billions of lives. But then the Mishhu grew less interested and there were these open territories and the Lorath scooted in. So now we end up with the YSE finding the Lorath in the territory it sees as its own.
By YE 34, Yamatai was too tied up in the Second Mishhuvurthyar War to do much else other than send out a token task force. Yamatai considered Lorath forces presence in YSE space to be an invasion and an act of war; but the Lorath had already been there for a year now under the pretense of protecting remaining UOC citizens from the NMX, so it was not the smoking cassus belli Yamatai needed to declare war. This created a powder keg that both sides had to avoid without sparking it. The moment a shot is fired it would turn into open, unrestricted, total warfare because Yamatai was still too imperialistic to suffer such an insult as an attack and the Lorath are similarly too prideful to surrender to people they see as un-people, and invaders, and totally against human rights.
Hanako was once again involved with the Lorath when, after finding enslaved Occhestian separatists in a grab bag of salvage (possibly pirated), she revived them and attempted to find the Occhestian separatist colonies in the galactic North. Instead, she encountered Lorath and the Lorath-allied Occhestians in the Occhestia System. The Eucharis was attacked by automated mines and opened fire in self-defense. LSDF forces, attempting to arrest the refugees, pursued the Eucharis despite several very grave warnings from the Star Army of Yamatai. The refugees were given asylum and two Star Army fleets were moved into position to attack the Lorath, bringing the nations to the brink of war.
In YE 36, after rounds of extremely high-tension negotiations at the International Relations Conference of YE 36, Yamatai entered a mutual defense treaty with Lor and Nepleslia, putting them on the same side. Yamatai and Lor even participated in a military soldier exchange program. Despite this, there are still undercurrents of distrust between the two nations.
In YE 38, at the International Relations Conference of YE 38, the Star Army of Yamatai's commander, Ketsurui Yui, argued that the Senate of Yamatai failed to properly ratify the treaty and therefore Yamatai was not a member. After the senate attempted to ratify the treaty and it was voted down, the YSE officially disavowed the treaty.