Norden Incident

The Norden Incident was a battle between the Imperial Northern Front and the Legadonia Entente Alliance Army. It is widely considered the start of the Great War.

Background
A long time ago, the Empire and Legadonia Entente Alliance had an unofficial dispute over the drawing of the national border. Legadonia used to claim the territory of Norden, which was under the Empire's control, but because the Londinium Protocol, which was mediated by the United Kingdom, allowed the Empire and Legadonia to resolve their national border dispute by compromising and recognizing a common national boundary (it was officially named a temporary demilitarized zone, and was confirmed by the executive government of both nations as a de facto national border), on the platform of international politics, neither parties argued over the ownership of the territory in question. However, a few coincidental events happened at the border recently. The border garrisons from both nations misfired accidentally, and firefights broke out because of some misunderstanding, causing the situation to become tense.

Legadonia Entente Alliance
The Legadonia was facing the sluggish economy, the widening wealth inequality, and the unemployment rate that refused to fall. Those finally led to the change of Legadonia’s policies. Due to the change in political regime and the installation of new leaders, the rise of nationalism demanded a huge shift in national policies. Since the Empire had territorial disputes to most neighboring nations, which formed an alliance against the Empire, the Legadonia's government expected the Empire would make some degree of concession. Once again, Legadonia claimed the territory of Norden as a sort of domestic propaganda. Later, they issued an ultimatum "Empire forces to leave their national borders within 24 hour" for the Imperial Army, in lieu of a declaration of war.

The Empire
The high command ignored the turbulent situation and judged it to be simply brinkmanship, expecting the neighboring nations such as Republic and United Kingdom would probably fear that their past efforts to help them would be lost, and would stop their military operations. Even during peacetime, a field army was garrisoned in Norden in the name of national security. After Legadonia issued the ultimatum, they followed the plans laid out by their country, mobilizing the cautious bureaucracy and military arm in preparations for the theoretical attack that might happen theoretically. In preparation for war, an ad hoc army was formed on top of that by mobilizing troops that weren’t part of the garrisoned forces. They gathered large amounts of resources and mobilized units, and even invited presses from other countries and permitted them to stay in Norden.

Invasion
One day in June, 1923, Entente Alliance sent the Imperial Army an advisory "Lower your arms and surrender to the nearest stationed army or evacuate promptly". Subsequently, Entente Alliance forces invaded Imperial Norden from several areas. As the response, the Empire declared war on Legadonia Entente Alliance. With armored units as the main corps, all Imperial forces were being mobilized. The Imperial Army built an air defense network that was properly set up, with air units and mages ready to intercept. Later, the Entente Alliance invading forces had been destroyed. Besides, when the Empire was about to switch to the offensive and pursue the enemy forces, a Legadonia aerial mage company broke through the Empire's air defense line, and attacked the Imperial mage observer. The Imperial mage observer got heavily injured yet successfully kept them from attacking the Imperial artillery position. This mage observer sooner was awarded the Silver Wing Assault Medal for her excellent performance.

Aftermath
Although the Empire had already defeated the invading forces, meeting their national defense objectives, the General Staff decided to mobilize the entire continental army, launching a major offensive against Legadonia. The Imperial General Staff attempted to finish off the Entente Alliance once and for all, and overrated the flexibility of the continental army. This decision finally led to a series of wars that are known as "The World War" in future.

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