The World War has swept through and caused havoc and destruction across the world; Europe is embroiled in a decades long war between the major powers of the world and a new threat attempting to take over - Lucifer's own army of the damned.
Beginning in the Holy Roman Empire in Northern Italy, a rift to Hell formed and enabled creatures to be unleashed from the inferno and into the world. In response to the demon army, the Holy Roman Empire requested help from their allies to fight the invading force. This invading force was much stronger than anticipated; taking control of Italy, Greece, Turkey, and Northern Africa from humanity.
Now with a strong foothold on Earth, they begun their war with humanity...
The allies of the Great War include a majority of the world's major powers, with a scant few cults joining forces with the demons. Of the nations fighting against the demons the major ones include the United Kingdom, the Holy Roman Empire, the United States, and France.
Earth's first trans-planar invasion is headed by Lucifer himself, Hell's forces utilizing many foes to keep humanity on their toes, using many different creature types outside of their demonic soldiers in order to enable different strategies and avoiding their demonic troops from being smote so easily.