The Finest Hour

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

Short Summary
The Empire strikes back! Tanya and her battalion commences their mission: blitz their way to end the war.

But will the war finally end in their own hands?

Another heroine is born, blessed with god's protection. Who might be this person?

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 that 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 had 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 destroy the enemy HQ, supply depot, and an unidentified bunker. Once done, they fly towards the coast to be picked up by an allied 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 Commonwealth tries to break a peace based on status quo ante. The Reich's leaders all ignore this proposal sensing victory. Also, the Commonwealth 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 Parisii. And she finds it odd that the defenses are weak. Meanwhile, at the port of Brest, General de Lugo (an 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.

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 the 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 in 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 Commonwealth 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 Russy Federation. To deal with this they are going to put Tanya (who is seen an apostle by the angels) against the Federation. 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 Commonwealth. This proves Mary was the chosen one.

Since the war doesn't end and the Free Republican Army is now in Southern Continent, the General Staff decides to send an expeditionary army to Southern Continent to see if they can end this war. However, one major problem is that apart from the small northern stretch in the baltic sea, they have no major port or naval supply system. As such, they can only send out a small force that would be re-supplied by a small ally named the Ildoa kingdom. The commander of this expeditionary army that is comprised of 2 divisions of worth of light infantry, mobile artillery, tanks and mages is Romel. And the 203rd Aerial Mage Battalion is sent to help him out.

Chapter VI: The southern campaign
Tanya and Romel work well together and become friends. After committing to what Romel (Rommel) is famous for, maneuver warfare, the imperial army is quick to gain ground and wins. De Lugo counters by trying to go around them and cutting them of from their ally Ildoa but Romel counters by taking one of the french ports instead. Intelligence agents embedded in the wandering nomads in the desert report that despite them taking a new port, their supplies are starting to run low. De Lugo comes up with a plan to draw out Romel by spreading false intelligence that the French army intends to settle this by marching upon their port in divided units, which would in turn draw out Romel to do what he does best and that would allow them to ambush him.

Romel hears of this intelligence and considering the lack of water his troops have, he decides to go for it. But on the eve of battle, Tanya is not convinced and requests permission to scout with her battalion. Romel, none the wiser about what could happen, lets her go without a care or thought while he prepares to go out. Tanya takes to the sky and splits up her unit to search the neighbouring area. But all of the units report back that they haven't seen anything. Tanya immediately realizes that the enemy has already reunited and prepares to attack.

Although Tanya's warning did manage to get through in time, the republicans were still close enough to get on the attack and pin down Romel and his forces. Tanya, after contemplating the many different ways to get out of combat and determining which would result in a firing squad, decided to attack one of the flanks of the republicans. De lugo gets a report while in the forward HQ (in the centre-rear of the republican lines) that a regiment of mages has appeared on the right flank. The Republican command immediately denies the report, stating that there is no way that their scouts and intelligence could have missed an additional regiment of mages. But when further reports come in about how several mage companies are dropping like flies, they believe it.

De Lugo responds by sending the main mage force from the front to the right flank to counter the mages but then he gets a terrible message, the mages are coming straight for HQ. Tanya noted that mages mainly rely on their magic senses rather than their eyes when their travelling, as such, it is easy to get around one another if one side isn't paying attention. This subsequently allowed them to do a strafing run on the republican HQ, destroying their equipment and barely missing De Lugo. When Romel hears of this, he bursts out laughing at the mere thought of Tanya and her skill. He subsequently put most of his forces on the left flank and manages to break through using maneuver warfare. This in turn forces De Lugo to call for retreat, giving the win to the imperials.

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

That ends 400+ pages of vol 3.

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