This is a scenario where Gallia attempts to gain a huge military and economic strategic advantage over Sirius without declaring war on Liberty and Rheinland. They do this by invading the Sigma systems, effectively declaring war on the GMG.
Here is a summary of events that lead up to this:
Bretonia succumbs to the military and industrial might of Gallia, and the New London government sue for peace.
Queen Carina condemns the peace treaty drawn up by Gallia ambassadors and calls for the war to continue.
Another year of war ensues and Carina is eventually deposed, and both the Royal Families of Bretonia and Kusari vow to continue the war in exile. The Queen goes to Liberty and the Emperor goes to Rheinland.
Bretonia sues for peace and accepts the harsh terms of the loss of all its Tau assets, Glasgow Outpost and LD-14 under the pretense of "peacekeeping operational bases".
Kusari lays claim to Planet Harris and negotiates with Gallia in exchange for the planet and local resources, allows military access and logistical support to Gallia Royal Navy for 40 years.
The Tau Accords are signed, Planet Harris is handed over to the Republic of Kusari and the Gallic Royal Navy sets up positions in Kyushu and Honshu.
Gallia wanting to strangle Sirius without attacking either Liberty or Rheinland directly, initiate operations to conquer the Sigma systems via military action.
Liberty and Rheinland condemn the actions of Kusari for allowing Gallia to operate with impunity and without restriction. Both Houses totally embargo Gallia, and Kusari.
Bretonia falls into economic and social collapse, rumours of revolution fills the streets, the BAF and BPA struggle to keep peace.
Liberty and Rheinland sign the "Treaty of Republics" and agree to a peace treaty of necessity and lift all trade embargoes between each other.
Gallia sends military units into the Sigma systems and declares the systems now a protectorate of The Kingdom of Gallia, refusal is seen as a criminal act.
Naturally GMG respond with its own military action and skirmishes with the Gallic fleet.
The Outcasts respond to this shift in power by sending large fleets to engage the Gallic fleet, leading to losses on both sides. Outcasts and GMG hold talks of 'alliance of necessity'.
Bretonia government collapses. Radical left wing government takes power.
Rather longer than I planned. But essentially, Gallia and Kusari have a boggled alliance, but really Kusari is now a slave to Gallia. What would happen do you think if Gallia does take control of the Sigma systems? What if Gallia loses the fight and is forced on the back foot? Would Rheinland and Liberty take advantage? Or is Kusari playing dead to catch the Gallic fleets by surprise and betray them?