The Great War, (Eurasian: Bellum Magnum; Lyrian: großer Krieg; Arveyran: Combatimenta magnifica), also known as the First World War, was the most destructive conflict in human history, claiming the lives of 65,000,000 people.
The war started on September 3rd, 1936, when Duresia and Eurasia invaded Reisal. The war itself began in earnest when Newellia and Zackalantis were invaded in 1937 and 1938, respectively. Following the complete annexation of those areas, the war itself became more an anti-partisan action, until the eventual collapse of the Duresian Republic which was a result of the Duresian Civil War, after which Eurasia was forced to abandon its conquests due to growing civil unrest. The Great War ended on May 8th, 1947, when the Eurasian Emperor Marinus officially withdrew all Eurasian forces and declared the war at an end.
As the war neared its close, the civil war in Duresia fractured Duresian claims in the occupied territories, with many descending into warlord-ruled petty fiefdoms. Eurasian forces attempted to maintain order by taking over the nominal administration of the territories, but it was highly ineffective. Due to standing order, many Duresian Republican forces loyal to the First Republic joined with Eurasian forces in the regions, whilst those who allied themselves with the monarchists and rebels fought both sides.
This had a profound effect on the continuation of the Omnestacita, as Eurasian forces attempted to hold all camps in both countries, which failed due to constraints on manpower. The Eurasians liquidated many camps, and bombed those they couldn't. By the time of the war's end, many thousands of Duresian republicans were fighting under Eurasian forces, and Marinus granted them all amnesty, which allowed for untold numbers of war criminals safe-haven in Eurasia.
The Tarajani Civil War is sometimes considered by historians to be a collateral part of the Great War: indeed, although the conflict was already under way when the war began, it only took the aspect of an all-out clash after the Bipartite forces completed their occupation of Zackalantis. The Tarajani Republican forces concerns about a possible invasion from the north, indeed, opened a window of opportunity for the monarchists led by then-Crown Prince Conrad to begin their first offensive on Tarajani soil. At the end, the Bipartite never crossed Tarajani borders, but Duresian monarchists also fought at the side of their Tarajani fellows until definitive victory was achieved in 1944. As a consequence, Tarajani volunteer units fought during the Duresian Civil War, and the newborn Kingdom also supplied Duresian monarchists with supplies and weapons (often remnants of those used by republican forces).
The outcome of the Great War was largely a disaster for the conquered. Newellia was internally destabilized, allowing it to quickly fall back under Eurasian influence, and as a direct result it was partitioned again in 2015. Reisal was absorbed into both Eurasia and Duresia relatively peacefully, and never experienced internal rebellion against the occupiers. It remains under the control of Eurasia and Duresia to this day. The UKZ was liberated, but even to this day suffers from internal disarray and is still feeling the aftershocks of the war.
However, the war did result in many changes in the aggressor nations as well. The Duresian First Republic collapsed and the monarchy was reinstated, Laurentius III of Eurasia was forced to abdicate to prevent the total collapse of the imperial state, and his successor, Marinus of Eurasia, instituted a number of liberalizing reforms which are still in effect to this day. Arveyres saw itself gain further autonomy from the Eurasian state, and New Tarajan saw itself reasserted as a global player. To date, the Great War was the costliest, most deadly conflict in human history.