On April 1, 1935, Vale and Vacuo jointly invaded Azimuth. They were met by a force of over 300,000 Azimuthian soldiers and over 750,000 troops from Mantle and Mistral. Their initial offensive was stopped on April 12, 1935, after they gained a measly 32 miles of land. On April 25, a counteroffensive saw MM Powers forces tear Vale and Vacuo apart by creating a thin stretch of MM-controlled land that stretched from the Azimuth-Vale-Vacuo border to the Northern coastline. This land, just 4 miles wide in some parts and 12 miles wide at its largest, became known as "No Man's Land." The Vale-Vacuo front changed little for the next 2 years after this event.
By July 12, 1935, the entirety of Lindwurm was under MM control, the MM Powers having crushed Volos, Stadia, and North Dragonica. On the Alidade-Mistral front, the war had been fought to a draw as Alidadian forces, assisted by a Vacuouan Expeditionary Force and a Valeanian Expeditionary Force, kept Mistral at bay following the Battle of Kumawara, which saw a total death toll of over 65,000 soldiers on both sides over the span of 4 days of fighting.
Until 1937, the war was basically a standstill. Vacuo and Vale kept hitting hard at MM Powers forces in Azimuth and No Man's Land, but they could not break the siege due to the strong naval forces of Mistral and Mantle. At the Alidade-Mistral front, costly battles began to wear the patience of Allied Powers forces thin, and a mutiny in January of 1937 by Vacuouan forces led to punitive punishments by their commanders.
On June 12, 1936, Allied forces invaded the Mistralian island of Menagerie, the traditional homeland of the faunus race, with a force of over 1,000,000 soldiers pulled from every country on their side. They landed at the Western half of the island, and though they took control of extreme Western end, they found it difficult to go beyond the coastal regions due to the strong defences of the MM Powers. An offensive attempted by the Allied Powers starting on November 10, 1935 proved to be a complete failure and the deadliest battle of the war. From November 10, 1935 to January 6, 1937, when the offensive was called off, a total of 2,000,000+ combatants on both sides plus civilians were killed. On January 20, all Allied forces evacuated Menagerie.
The trauma of the Menagerie Campaign is believed to have finally pushed some within the Vacuouan government over the edge. On January 25, a telegram was sent to the King of Vacuo. In it, it read that if he did not contact the MM Powers to negotiate a changing of sides, he would be overthrown and killed. He reluctantly agreed. On March 1, 1937, a telegram was sent to Mantle and Mistral of their intentions. They were overjoyed, and from there, the 3 began plotting.
Meanwhile, events in Azimuth caused the another spanner to be thrown into the cogs of the war. The Allied Powers reluctantly backed a coup d'etat and subsequent civil war started by Collectivist rebels that sought to create the first Collectivist state in Remnant. It started with the mass murder of the entire royal family, including the Prince, on February 18, 1937. By March 3, 1937, Mistralian and Mantillian forces had taken command of all of the anti-Collectivist forces in Azimuth, while Collectivist forces battled with Allied support. On April 3, 1937, Azimuth pulled out of the war for good with the victory of the Collectivists, breaking an agreement they made with the Allied Powers. Even so, no one managed to fight Azimuth for the rest of the war. The Vale-Vacuo-Azimuth front continued on, though, since Mistral and Mantle had constructed a new port city on the northern coast of No Man's Land.
On April 28, 1937, the guns on the Vale-Vacuo-Azimuth Front and the Alidade-Mistral on the MM Powers side went silent suddenly. Valeanian forces were confused as to why. Then, on April 29, their Vacuouan Allies suddenly turned on them.
In the Betrayal of the Valeanians and the Alidadians, Vacuouan soldiers fired on their former allies. They showed no mercy, either. Even Mistral and Mantle were shocked at the level of brutality Vacuouan forces showed against their former allies. They tortured and executed male POWs and male Valeanian civilians, raped female POWs and female Valeanian civilians, purged their own forces of all pro-Valeanian elements, and often desecrated their bodies by publicly parading them. From April 29 to May 27, a total of 1 million Valeanian soldiers, 300,000 Alidadian soldiers, 500,000 Vacuouan soldiers, 250,000 Valeanian civilians, and 350,000 Alidadian civilians died.
The brutality of the Betrayal caused a massive uproar in Mantle, which had been long beset by ethnic tension between the Mantillians, who composed 35% of the population yet ruled the country, and the Atlesians, who composed the majority. On June 12, following 13 days of massive protests across the kingdom, a coup d'etat overthrew the Mantillian King. In his place came a general who was of Atlesian descent who was crowned as the new King of Mantle. On June 20, Mantle switched sides and declared war on Mistral and Vacuo.
Now, the war was Mantle, Vale, North Dragonica (in exile), Stadia, Alidade, and Volos (in exile) as the Allied Powers versus Vacuo, Mistral, Dragonica, Hachure, and Meridia as the Vacuouan Powers. Azimuth remained neutral.
By October, the war had once again come to a stalemate. The Vale-Vacuo front was stabilized, and the Stadia-Mistral front was stabilized as well.
Technology played a large role in the brutality of the Great War. Because no major wars had been fought in Remnant since the 1880s, many battlefield leaders had no idea of the brutality new weapons produced. They were still using battle tactics from 100 years prior against weapons such as machine guns, chemical weapons, tanks, and aircraft. The 3rd Vale-Vacuo War had seen these weapons' first test in combat.
The machine gun, invented in 1822 as a hand-cranked rotating gun and then perfected into a recoil-operated single-barrel version in 1860, was first widely used by Vacuouan forces during the war. Their effectiveness resulted in the other countries of Remnant adopting them. By the Great War, handheld submachine guns, also known as machine pistols, had become a reality as well.
The first modern chemical shells were produced by Vacuouan scientists and weapons developers in 1866. In the 3rd Vale-Vacuo War, they were used with great efficiency. However, the only agents that were used were tearing agents, not choking or blistering ones. By the Great War, however...
Tanks had been developed around 1900 by Mantillian forces. After their prowess and capabilities were shown off at the Remnant Exposition Fair of 1908, other states began to develop their own. By 1935, tanks had seen combat in small conflicts, but never in major wars.
The first powered aircraft in Remnant's history was flown on June 1, 1878. During the 3rd Vale-Vacuo War, rudimentary biplanes flew as spotters, though some forward-thinking pilots began to carry pistols and grenades with them so they could shoot enemy spotters and drop explosives on enemy troops. After the war, aircraft technology began to improve. In 1910, the first jet aircraft was flown at an exposition. Even so, piston-engine planes remained dominant until the Great War due to a lack of want to develop the technology further, in part due to lobbying by the railroad industry.
The Great War changed battlefield tactics and weaponry forever in Remnant. Despite this, it would not be until the Faunus War 50 years later that many outdated ideas and traditions finally bit the dust, such as nobility taking charge as officers, horseback cavalry, swords as sidearms, and others. The fact that soldiers in the Great War often carried both modern semi-automatic rifles and longswords, and that cavalry would charge into battle armed with both lancers and pistols was curious. The fact that both of these seemingly anachronistic occurrences were common into the 1990s, when it became even more anachronistic with even more updated weaponry such as missiles and assault rifles, was embarrassing.