Treaty of Londinium

The Treaty of Londinium was a treaty between the Empire and the Legadonia Entente Alliance under the mediation of the Commonwealth. The treaty set the line of control between the two countries in the disputed territory of Norden, but the intentional ambiguity in the terminology, meant to make the treaty acceptable to both sides, ended up leading to a new conflict.

The Treaty
While the novel didn't list the contents of the treaty line-by-line, the following were mentioned:
 * The treaty divided the disputed territory with a demilitarized zone.
 * The treaty guaranteed the administrative rights of the two countries' respective zones.
 * The treaty did not address the sovereignty of disputed territory.
 * The treaty, as well as the demarcation line, are called "provisional".
 * The treaty's ambiguities were intentional to respect Legadonia Entente Alliance's claims.

Failure of the Treaty
Due to the economic downturn, widening wealth gap, and troubles with unemployment, the government of Legadonia Entente Alliance turned to nationalism, and in an attempt to boost their election, send their army across the demilitarized zone to occupy all of Norden. However, they didn't think the Empire to respond militarily, and the ill-prepared Legadonian forces were routed by Imperial forces.

After the Legadonian forces fell back, some members of the Imperial General Staff, including Brigadier Generals Zettour and Rudersdorf, wanted to preserve the line settled by the Treaty and were reluctant to advance into undisputed parts of Norden, but their superior Lieutenant General Ludwig, seeing that no other countries mobilized in response to the incident, decided to push into Legadonia proper, finalizing the death of the treaty.

Real-life Inspiration
The fictional treaty was partially based on the London Protocol of 1852, according to footnotes of Volume 1. It was signed after Denmark won the First Schleswig War to guarantee the Danish king's possession of his fiefs in Germany.

King Frederick VII of Denmark was also the duke of Schleswig (vassal of Denmark), Holstein, and Lauenburg (sovereign members of the German Confederation). However, he was childless and the last member of his line, and the difference in succession laws meant that Holstein and Lauenburg would not be inherited by the next King of Denmark. This protocol changed their succession law so that they will remain under Danish control after his death, in return, Denmark must not undertake any measures to make Schleswig any closer to Denmark than Holstein already was.

Compared to Youjo Senki, Denmark was similar to the Empire (guaranteed actual control with nominal concessions), while the German Confederation was similar to the Entente Alliance (nominal preservation of legal rights).

However, the London Protocol had a different outcome. After the death of Frederick VII, the new king Christian IX ratified a new constitution that would create a joint parliament of Denmark and Schleswig. This immediately triggered a war with Prussia and Austria, as well as the German Confederation as a whole. As Denmark violated the Protocol, they received no foreign help, and the joint Austro-Prussian army controlled the entire continental Denmark by the end of the war. All three duchies were ceded to Austria and Prussia. Schleswig-Holstein remained a disputed territory until it was partitioned after World War One. The later Nazi Germany didn't dispute the partition and maintained the line even during their occupation of Denmark.

Reference
Deus lo vult - Chapter I The Sky In Norden.