It started with the end of the Allef.
The war had gone on for a hundred years. Collectively it was known as “the Garden Conflicts”, because at various points in its history empires would leave and later rejoin the fighting. Looking back on it now, no one truly left the war; they merely stopped fighting for a relative brief moment to catch their breath and replenish their losses. But usually within a generation, empires would be back looking for justice, or vengeance, or resources, or whatever it was they were fighting for this time.
After a century of conflict, the empires started to get desperate. Billions of sentient beings had lived their whole lives knowing nothing but war. Only the Granos and the Narasheem could remember a time when there was no war, back before the empires even knew the others existed. Many empires sought out new weapons and technologies; some edge that could bring this conflict to a close once and for all. The Uranar tried it first with Gesalin Gas, which they unleashed on the Republic of Kel. The death of Iasalo, three hundred thousand humans, drove the Republic into a frenzy. The Narasheem and the Granos weren't too thrilled with their neighbor either, and formed a truce with the Republic. That phase of the war ended in a stalemate; the Uranar couldn't use their Gesalin gas on any of their enemies' worlds, and the trio couldn't invade the Reaches for fear of losing their forces to Gas attack.
The Republic researched “Jump Cannons”, mass drivers that could fire projectiles through localized jump gates. The Granos attempted to create supersoldiers through bio-tech research. The Narsheem sought out the power to weaponize solar flares to bathe fleets and planets in electro-magnetic pulses. Each empire had something that they were working on that could result in the utter annihilation of their enemies, but it was the Allef that made everyone wake up and see what the century had wrought.
The Allef, besieged by the Naguli and the [[Drakon Clans]], and abandoned by their allies in the Republic, were desperate. The Allef Domain had shrunk, and they were reeling. The Naguli had conquered most of the Domain’s former worlds, while the Drakons raided the rest until there was nothing left worth taking. With their backs against the wall, and no help in sight, the Allef attempted one last gambit to save their people and stop the war; they succeeded with the latter goal.
On that fateful day, a Naguli invasion fleet appeared over the Allef homeworld. As they approached, the Allef engaged their secret weapon; a gravity singularity bomb. The bomb detonated in front of the Naguli fleet, just as the main command ship was arriving. The gravity singularity formed at the same time as the massive command ship’s jump gate. The two fields interacted, catastrophically. When the blast faded, the Naguli fleet was gone, but most of Allef was gone with it. Over 60% of the planet’s mass, the area closest to the singularity, was drawn in. It was as if a giant entity took a massive bite out of the planet. Allef’s atmosphere destabilized, it’s gravity was thrown off by its loss of mass. What was left of Allef shook itself apart over the next ten days.
The suicidal destruction of Allef shook the other empires to their foundations. They saw the lengths that a once powerful empire took to win the war and were shocked into action. Rescue ships from several empires headed to Allef to try and save anyone they could. Even so, only ten thousand Allef were rescued from the planet that once held three billion.
The death of the Allef greatly disturbed the other empires. They took a hard look at themselves, their people, and the effects the war had on the entire Stellar Garden. This event prompted the Republic to negotiate with the other empires. The Granos Territories responded first, but eventually the Narasheem worlds, and the Uranar Reaches entered negotiations. Even the Naguli Empire came to the table. The Drakon Clans ceased their attacks as they evaluated what the empires were doing. After months of arguing, deal making, negotiating, and eventually peacemaking, the League of Allied Worlds were formed.
And all it took was the death of an empire.