For a similar conflict in the main canon, see the Third Russo-European War.
"Einstein was a fucking liar."
-Anonymous, on WWIV
World War IV (also known as "The Last Great War", the "TNA-MekIntern War", and others) was a large-scale world conflict fought between the nations of the Tandi-Nama Accord and Mechanocratic International. With hundreds of millions of casualties, trillions of dollars' worth of U.S. dollars' worth of property damaged or destroyed, it is considered the most devastating conflict in human history, seconded only by World War III that saw billions of casualties and a complete restructure of Earth's geopolitical foundation. The war was fought primarily in Europe and the UDAP, though the peak of the TNA offensive would see fighting in the fringes of Mecharussian territory, and surprise Mechanocratic attacks on Imperial territory would see several prominent battles fought in TNA territory, in places such as Japan and the supercity of The Hub.
Nearly all agree that the war itself began with a miscommunication; British V-Bombers were dispatched [INSERT REASON HERE] to launch a completely unauthorized strategic first strike against UEM defensive installations in Western Europe, as dictated in TNA war plans regarding an invasion of Europe, in early 2156. When high command received word of this, it was determined that it would be too late to recall them without causing irreparable damage. Instead, a cabal of extremists within TNA High Command, headed by Frenkish Grand Marshal Ulysses Turner, [INSERT OTHERS], managed to convince the others that a first strike against the Mechanocracy would be in their best interests, and the bombers had a high chance of success if they were allowed to continue with their mission instead of being called back.
However, disagreement flourishes when considering the reasoning leading up to the TNA first strike. Many point to the recent Mechanocratic domination of continental Europe; a major power-play by Mekrus against the New Frenco Empire, who had almost always held high influence on the continent since either's inception. Many humanists cite the treatment of Western Europeans in particular, who (for the most part) remained opposed to Mechanocracy and as such, were often subject to brutal oppression, often supporting the war's means to "liberate" Europe. Pragmatists, on the other hand, suggest that the loss of Europe was a tipping point in the Second Cold War, and the TNA had little choice than to gather and deploy its vast martial assets in an act of "preemptive self-defense".
Others actually point to the common anxiety shared by the political entities of Earth following the discovery of the Sidhae. Though their existence was preserved by the Mecharussian government following the Lenin Affair, by the failure of the Sixteen July Pact (with a small cell of Imperial field agents actually aiding the Mecharussian government in the battle), their existence was common knowledge to the greater Solar System, and many (correctly) hypothesized that the Mechanocracy hoped to use the Sidhae to their advantage over geopolitical matters on Earth. Having been the first to discover the ultra-advanced alien empire and gain a headstart on relations, the nations of the TNA felt threatened, and sought to take away that edge by whatever means necessary.
Still, some just consider WWIV the logical conclusion of a decades (centuries, by some definition)-long cold war, dismissing other theories with the statement that "it was bound to happen one day". Many point to Ulysses Turner, notorious for his staunch paranoia regarding Mechanocratic communism, as the ticking time bomb that eventually would go off.