Tribble 501...
(apologies ahead of time for this being so slapdash -- this is a partial reconstruction of some of my notes from files on my now nonfunctional computer, so...)
Operation: Nightfall
In this version of things, almost everything up to Kerensky's Exodus happens they way things do in the main version -- except Minoru (and Jinjiro) Kurita and Kenyon Marik form an alliance to take Terra, and Blake is a whole lot more unstable. Minoru dies in combat against Terran forces -- so the Kentares Massacre doesn't occur -- at least, not how it happened in the Prime Universe. As part of the alliance, the one who takes Terra becomes First Lord, while the runner up becomes Commanding General.
Blake sends out a demand for assistance when it becomes clear that Terra will fall -- but his messages are delayed by traitors within. Similarly, traitors within allow the Kurita-Marik forces to escape harm from those still functioning Space Defense Systems.
Meanwhile, Davion, Steiner, and Liao hammer out a temporary alliance of convenience to try to stop -- or at least mitigate -- the damage -- even managing to bring some Taurian, Canopian, & Rim Worlds forces into the attempt. The Alliance sends forces to several planets near Terran space -- the Capellans to Liao & (space-holder till I figure out one), the FedSuns to Chesterton and a couple others nearby, and the Lyrans to Skye, Summer, & probably another.
Each of the rulers are with their troops -- the Chancellor on Liao, the First Prince on Chesterton, and the Archon on that unidentified Lyran world....
When it becomes obvious to the increasingly paranoid Blake that Terra will fall, and due to the whispers saying the other states are joining his enemies, he launches Operation: Nightfall -- dozens of WarShips & JumpShips carrying DropShips loaded with all sorts of nuclear, biological, and chemical weapons arrive over most of the capital & sub-capital worlds in the Inner Sphere -- oh, and of course, the worlds the Davions, Liaos, and Steiners have gathered their relief forces...
This news does not make it to the Kurita-Marik forces, though -- just before they manage to arrive at Terra, Blake and his inner circle flee toward the one (in this universe) "Hidden Five" world (Odessa/Gabriel Station) that Blake & company have already claimed. First Lord Marik and Commanding General Jinjiro live just long enough to regret not bringing the Space Defense Systems back online when the Revenge Fleet arrives to return the favor...
Once everything has settled, 90+% of the Terran Hegemony worlds have been sterilized, while most of the capital & sub-capital worlds of the Inner Sphere & near-Periphery have also been rendered lifeless...Blake dies on Odessa when that world is nuked to oblivion...
Of the various heads of state, only Steiner, the Warlord of Rasalhague, and the Andurien Duchess survive. Both the Draconis Combine and Free Worlds League splinter -- Andurian & Regulus declare their independence almost immediately after the death of Kenyon on Terra -- similarly Arkab and Rasalhague announce their own intentions.
By the year 2800 most things have settled -- cadet branches of the Liao and Davion families have selected new leaders, and that 'temporary alliance of convenience has largely held up.
Andurien has merged peacefully into the Capellan Confederation (a political marriage, of course), and together claimed most of the territory around Oriente -- and now have a border touching Regulan and Steiner space.
Regulus claims most of the former FWL territory (its traditional areas, Gibson and surrounding areas, the Regulan Free States & Rim Commonality, chunks of the Marik Commonwealth & even some of the worlds out near Tamarind, the Abbey District, & near enough to the Lothians & Illyrians...several small states lie between Lyran and Regulan territories...
In Combine space, as mentioned already both Rasalhague and Arkab exist as strong states -- there is also a Pesht Supremacy, and two small, mutually hostile states -- one based on or near New Samarkand, the other on or near Galedon -- who both claim to be the only legitimate realm in Combine space, and both claim all the worlds there...
-- this is about where I ran out of memories in that reconstruction parts...
(this idea grew out of an idea 'what if the Dragoon Compromise had been more than just the Wolves & some Scorpions? -- what if it had been a coalition of roughly half the Clans?'
and I really wanted to take Terra off the playing field...)
If I remember more (and if anyone who like me to add that stuff) or get my machine working so I can access my notes, I'll add more...