Dabit deus his quoque finem

Dabit deus his quoque finem is the 4th volume in the Youjo Senki Light Novel series.

Short Summary
The day of the Empire faced another crisis.

Russy Federation start to mobilize their troops.

Who will win the Eastern battle? Who will succeed?

Will Tanya finally find her own peace and win against Being X?

Chapter I: Long range reconnaissance mission
Tanya and her 203rd battalion are sent to the Reich-Russy Federation border. They are conducting a secret reconnaissance mission inside Federation borders because the General Staff has intelligence that the Federation is planning an attack.

However expectedly, the war starts with the Federation invading the Reich's border by surprise. The main reason, it turns out that Josef, the General Secretary of the Federation, had a message from the angels that an apostle of God was coming to destroy the Federation. So Josef decided to strike first.

Chapter II: Goodwill visit
After seeing them fire first, Tanya decides to take action and attack the Federation. While the 203rd knocks out Federation artillery and supply depots, she realizes that there are no mages.

The 203rd then decides to pull another Dacia and attack the capital, Moskva. This action will also complicate the Federation's invasion of the Reich so the General Staff agrees to this.

Unfortunately for Tanya, while she is having fun at Moskva, the number 2 man in the Federation, Loria (Lavrentiy Beria), spots her and falls in love with her.

Loria vows to do everything in his power to capture her, break her, and ravish her. So he decides that the Federation needs to release the prisoners from the lageri, particularly the mages (who were loyal to the previous Imperial regime), and win this war.

Unfortunately for the Reich, that means the Federation (which was purging itself of competent officers in the name of the Communist Revolution) will now be a formidable foe.

Also unfortunately for the Reich, Tanya's attack makes any possible ceasefire with the Federation impossible. It angers the Federation leaders, and it raises war fever in the Reich so much that the General Staff can't go against popular opinion.

Chapter III: Magnificent victory
The Federation army has a big advantage in numbers but they are completely lacking in mages. Their invasion stalls. The fact that the mages can destroy the Federation supplies behind the lines hurts them.

The General Staff is now convinced that they can draw in the Federation army and win.

Chapter IV: Reorganization
Tanya is recalled to the capital. Zettour is angry because she did "too much," and so she is transferred west to the mainland of Francois. She is given time to come up with new tactics.

The enemies of the Reich, namely the Federation, Commonwealth, Unified States, step up their game. Federation agrees to Loria's plan to release the prisoners from the lageri and use them to fight against the Reich. The Commonwealth begins to buy tons of armaments from the US.

Mary Sue is now in the mainland of Commonwealth. She is training as a member of a US volunteer expeditionary force comprised mostly of former Entente citizens and their descendants.

Chapter V: Battle of Dodobird
The 203rd is part of an aerial war against the Commonwealth. In one of the engagements, Tanya encounters Mary Sue and the volunteer expeditionary force. But despite Tanya's best efforts, Mary Sue doesn't go down. Mary Sue also sees that Tanya is using the submachine gun that she gave to her father and is enraged. Her rage mode activates.

Chapter VI: Operation Door knocker
Tanya is recalled to the capital to write up reports. One report suggests that an adhoc combined army kampfgruppe, a temporary and smaller version of a brigade, be formed.

While Tanya is now convinced that she can now stay in the rear and write up more reports, Zettour decides to give Tanya a combat group. So now Lt. Col Tanya must form a kampfgruppe in 5 days. She will be deployed in 10 days then fighting within 21 days.

Meanwhile, at Moskva, Federation leaders are discussing what needs to be done after their invasion army's defeat. Loria suggests that the Union withdraw and draw in the Reich's forces to fight a long drawn out war. The Federation should fight more battles in their cities, where the mobility of the Reich's army is limited, and try to bleed the Reich dry with guerrilla tactics. The Federation can withstand the losses but the Reich cannot. Also Loria begs Josef that he wants Tanya as his prize. Josef agrees to both.

That ends 470+ pages of vol 4.

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