Dabit deus his quoque finem

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

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

The war starts with the Union invading the Reich. It turns out that Uncle Joe had a message from the angels that an apostle of God was coming to destroy the Union. So Uncle Joe decided to strike first.

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

The 203rd then decides to pull another Dacia and attack the capital, Moscou. [This part is hilarious so I won't cover it too much in detail. I am sure it will be depicted in the Manga.] This action will also complicate the Union's invasion of the Reich so the General Staff agrees to this.

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

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

Unfortunately for the Reich, that means the Union (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 Union impossible. It angers the Union 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 Union 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 Union supplies behind the lines hurts them.

The General Staff is now convinced that they can draw in the Union 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 notFrance. She is given time to come up with new tactics.

The enemies of the Reich, namely the Union, UK, US, step up their game. Union agrees to Loliya's plan to release the prisoners from the gulags and use them to fight against the Reich. The UK begins to buy tons of armaments from the US.

Mary Sue is now in the UK. 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 UK. 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 Tommy gun that she gave to her father and is enraged.

Chapter VI: Operation Door knocker
Tanya is recalled to the capital to write up reports. One report suggests that an adhoc combined arms combat group (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 combat group in 5 days. She will be deployed in 10 days then fighting within 21 days.

Meanwhile at Moscou, Union leaders are discussing what needs to be done after their invasion army's defeat. Loliya suggests that the Union withdraw and draw in the Reich's forces to fight a long drawn out war. The Union 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 guerilla tactics. The Union can withstand the losses but the Reich cannot. Also Loliya begs Uncle Joe that he wants Tanya as his prize. Uncle Joe agrees to both.

That ends 470+ pages of vol 4.

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