Plus Ultra

Plus Ultra is the 2nd volume in the Youjo Senki Light Novel series.

Short Summary
The other states have started their moves. Will the Empire stand alone and win this war?

Tanya is carrying her own battalion in this desperate crisis. Will she live happily ever after?

Chapter I: The Dakian War
The 203rd Aerial Mage Battalion attacks the capital of Dacia.

Chapter II: Norden I
203rd Aerial Mage Battalion transferred to Norden.

Chapter III: Norden II
The General Staff decide to end the war in Legadonia by landing troops behind enemy lines, and cutting off their supply lines to the front. Tanya also independently reaches that same conclusion, but she is thinking from her past precedence, ie MacArthur's Inchon landing. Tanya's battalion takes part in this operation as she engages Anson Sue the second time. They fought earlier in Tanya's first engagement in 1923 from which she got her Silver Wings Assault Medal.

Chapter IV: The devil in the coast of Norden
Anson Sue receives a submachine gun as a gift from his daughter, Mary. Mary Sue and her mother fled to the Unified States because the Norden forces are in disarray.

The Entente government decides to continue the war by going into exile (flee across the North Sea to Commonwealth). The 203rd Aerial Mage Battalion is sent on a recon mission and encounters a fleet carrying the Entente government in exile. Anson Sue attempted to defend the fleet but Tanya kills him and stole his submachine gun, and then attacks the one ship carrying most of the government in exile.

On the way back to base, Tanya and her battalion spot an unknown submarine and orders for it to be boarded. The submarine ignores the order, so Tanya fires warning shots at the sub. And the ship belonging to the Commonwealth sinks.

The Commonwealth intelligence service thinks there's a mole in their organization. Tanya doesn't know that she killed really important people and is worried that she failed in her duty to slow the fleet so that the Reich's navy could destroy the fleet.

Chapter V: The Rhine's demon
The 203rd Aerial Mage Battalion is transferred to the Rhine Front. Tanya is acquitted in a court-martial for the sinking of the sub, but this leads to more propaganda from Commonwealth.

Second Lt Grantz also joins Tanya's battalion at the Rhine Front. Their first mission is to sneak into enemy lines at night and take some prisoners of war for interrogation. In the anime, he had already joined the battalion.

Meanwhile, the General Staff debates Zettour's plan to entrap Republican Forces with a strategic retreat.

Chapter VI: Ordeal of fire
Arene rises in rebellion and Republican mages drop in to support the rebellion. Railway lines linking the Reich's forces at the Front with the Rear are cut. The 203rd Aerial Mage Battalion is ordered to quell the rebellion.

Tanya wiggles out of this by prioritizing the rescue of Reich's citizens rather than hunting down rebels (who declare that they will all resist to the end, thus technically terminating their legal status as civilians). In the end, when the 203rd Aerial Mage Battalion eliminates Republican mages by putting up magic shields, artillery shells then rain down on the city.

Chapter VII: Preparation for move forwards
The 203rd Aerial Mage Battalion is ordered to fight a long-range recon and combat mission to disguise the retreat of the main army from the Low Countries. This mission succeeds but at a huge cost to the battalion.

The General Staff also comes up with their version of "Operation Shock and Awe."

Side story: A borrowed cat
This is the Propaganda photo shoot that is at the very end of episode 2 in the anime. That ends 560+ pages of vol 2.

Reference
Thanks to u/MSG_Johnny2 who posted the summaries at r/YoujoSenki.