The Finest Hour

The Finest Hour is the 3rd volume in the Youjo Senki Light Novel series.

Chapter I: Open Sesame
Mary Sue is in Arkansas with her mother and grandmother. They learn that her father is now dead.

The General Staff's "Shock and Awe" operation decapitates the enemy by destroying their headquarters. Once the enemy is in disarray they can mop up the main army.

To do this the weakened 203rd Aerial Mage Battalion is ordered to select its best among the survivors. They are placed on rockets built by the mad scientist Schugel and sent to the far rear of enemy lines (despite Tanya hating this idea).

Chapter II: The Intervene, Which was too late
The Republican army's left flank moves into the Low Countries (the area that the Reich has pulled out at the end of vol 2). The Reich's political leaders are all up in arms because the industrial area of the Reich is now under threat.

Tanya's special forces land behind the Republican army's front lines and destroys the enemy HQ, supply depot, and an unidentified bunker. Once done, they fly towards the coast to be picked up by an ally submarine.

Once the Reich HQ hears the success of the mission, they blow a hole in the Republican army's right flank with sappers. Since the Republican army has no overall HQ it is completely in disarray and gets rolled over. [The author calls this the "revolving door" but basically this is a massive Austerlitz for anyone who happens to know military history.]

Meanwhile the United Kingdom tries to break a peace based on status quo ante. The Reich's leaders all ignore this proposal sensing victory. Also the UK intelligence service is now convinced that there's a mole. The unidentified bunker that Tanya destroyed earlier was a secret intelligence gathering bunker for the UK.

Tanya is on the submarine on her way back. She is thinking that as long as there is no "Dunkirk" like event they might win this war after all. And once she's flying over the battle zone to return to base, she is convinced that they have won a rousing victory.

Tanya's battalion is one of the first to arrive at the outskirts of Parisee. And she finds it odd that the defenses are weak. Meanwhile at the port of Brest, General de Lugo (anagram for de Gaulle) is gathering Republican forces and putting them on their naval vessels.

Chapter III: Operation an ark
The Reich's leaders are thinking of peace terms. Basically they want to expand their borders, get reparations, and they will demand the Republic to abandon some of their overseas colonies. [It's a Versailles Treaty in reverse.]

Tanya hears that the Republican navy is gathering at Brest under General de Lugo. And recognizes that this will be like that "Dunkirk" event. The problem is that the Reich's leaders all think the war is virtually over and wants a cessation of hostilities to negotiate peace.

Tanya objects loudly and even prepares to launch another strike at Brest, like she did earlier at the Republican HQ. But she is ordered to stand down. A ceasefire order is issued.

Chapter IV: How to use victory
Rest of the 203rd Aerial Mage Battalion soldiers is now at Brest and celebrating victory. Tanya decides to go to the capital to talk to Zettour to ask what is going on. There she finds out that everyone is also celebrating. She is the only who is despairs. She realizes the only opportunity to win the war had slipped away at Brest.

General de Lugo declares that the Republic will fight on in exile as the Free Republican forces. The UK also declares war on the Reich as an ally of the Free Republic and the surviving elements of the Entente forces.

Chapter V: Internal affairs
The angels are discussing events on this alternate earth. Some are happy that faith is up again but others are worried that atheism is spreading in the Union. To deal with this they are going to put Tanya (who is seen an apostle by the angels) against the Union. They are also going to pick someone who is already devout to be another apostle.

Mary Sue joins the US volunteer force to fight against the Reich and is sent to the UK. This proves Mary was the chosen one.

Since the war doesn't end and the Free Republican Army is now in North Africa, the General Staff decides to send an expeditionary army to Africa to see if they can end this war. The commander of this expeditionary army is Romeel. And the 203rd Aerial Mage Battalion is sent to help him out.

Chapter VI: The southern campaign
Tanya and Romeel work well together and become friends. In one of the battles, de Lugo is about to win by strategically enveloping Romeel. But Tanya pulls off what the Shimazu clan did at Sekigahara, an escape after a frontal assault, and rattles the Free Republican Army's HQ. This allows Romeel to break out of the envelopment.

This chapter ends with the Prime Minister of the UK declaring that future history book will mark this moment as the brightest moment of the Reich and the darkest moment for the UK. But the worst moment has now passed. [Meaning darkness will turn to light and light will turn to darkness and that the UK will ultimately win.]

That ends 400+ pages of vol 3.

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