The First Europan War was caused by friction between the Autocratic East Europan Imperial Alliance and the Federation over Ragnite resources.
The Industrial Revolution of the 18th Century led to skyrocketing demand for ragnite, pitting Europa’s twin giants in a race for resources. When the crown prince of the Empire was assassinated at the start of the 20th Century, tensions spiraled into an all-out war. Though the Imperial force employed a corps of newly developed tanks, hostilities soon became bogged down in fruitless trench warfare, and the two exhausted powers ultimately signed a cease-fire agreement.
During the war, Gallia’s military, led by General Belgen Gunther’s tank corps, foiled an Imperial invasion attempt.
Interesting premise that could easily be expanded upon.
Despite my recent article I am still surprised that the developers and writers went with a civil war story line for Valkyria Chronicles 2 and returned to the previous Europan War II conflict in Valkyria Chronicles 3 rather than examine conflicts that led to the second Europan War.
Source: http://valkyria.wikia.com/wiki/EW1