The Great Moroccan War (or 1st Great Eurasian War according to Japan, China, and the Ottomans). With killing fields across Eurasia, and violence in Africa and Oceania the war saw over 30 million dead from 1911 to 1914, and with ensuing famines and violence that followed the official end of the war nearly 20 million more die (mostly in China and the former Russian Empire).
The war was disastrous for a number of powers. The Germans had hoped to launch a quick decisive war while the French military was in the middle of reforms and the Austrians were engaged in a transition of monarchs while trying to deal with an increasing divide between Vienna and Budapest. Unfortunately a longer than expected siege of Ghent in April of 1911 ate heavily into limited German nitrate reserves. They managed to fight on relatively efficiently throughout most of 1911, but by the spring of 1912 had to chose between explosives and fertiliser. The Germans chose to rely on Russian food exports to make up the shortfall. Still, they were forced on the defensive, and during brutal winter fighting Brussels fell to Entente forces (mostly led by Canadians, experienced in winter combat). After Belgium fell the war saw a slow slog as the Entente pushed through Belgium towards Germany.
The invasion of Austria went much better for Germany, though they were dismayed to find Italy back out of it's previous commitments (Germany's advance grinding to a halt in Belgium left them with the impression the war was lost). Austria would never recover. Russian and Bulgarian forces led most of the offensive into Ottoman and Greek holdings, a front that went quite well (though Russian forces were often poorly armed and commanded). Istanbul was besieged by the autumn of 1911, Athens by the start of 1912's spring. Royal and French naval assistance was vital to both cities being held.
Central Asia was simply a disaster for Russia. The Tsar was convinced that India would rise up if Russia threatened British rule and launched a massively oversized invasion via Persia and Central Asia. Attempting to supply the nearly 250 000 men deployed to assault India overloaded the region's infrastructure, leading to food and ammunition shortages for Russian soldiers along with food shortages for locals. Desertion amongst soldiers and local unrest led to widespread banditry, and revolution in Persia. The Raj meanwhile was more than capable of defending itself, especially when news of hungry Russian soldiers attempting to live of the land and killing villagers who resisted pillaging in Afghanistan and the northern fringes of the Raj flushed the Indian army to over 2 million volunteers (in combat and non-combat positions). Most Indian forces were used in Afghanistan and Persia, though over half a million were deployed to defend the Ottoman Empire.
China would descend into a confused killing field. Russia supported the Emperor, while Japan (and by extension the Entente) supported the various republican elements. However a third anti-foreign faction (the League for Chinese Dignity) despised both groups. For most of the war the Entente was forced onto the defensive, barely holding Korea, a strip of the southern coast, and the regions surrounding Shanghai, however the Emperor and LCD fought one another with a ferocity that prevented major offensives against delicate Entente positions. The Japanese and Australians combined forces to liberate the Philippines (which had broken into open revolt not long after war was declared).
Africa saw relatively limited combat. The worst fighting was in the Boer Republics, which hoped to gain access to the coast, and perhaps more, by siding with Germany.
Canada and Mexico would both be frequently accused of keeping too many troops at home by other Entente nations, but the risk of American entry into the war as US merchant ships frequently pushed into Entente blockades was recognised by all as a serious risk.
The end of the war was decided by September of 1913 though. Strikers in Moscow were shot down by machine guns and the city exploded into rebellion. Soon Finland and Central Asia erupted into open rebellion. Military defectors and angry local peasants in Turkistan launched the League of Socialist and Anarchist revolutionaries, while Finland was a confused web of separatists and communists. Rebellion in the rest of the Empire soon followed, and harvests were disrupted across the nation. With the military called back rapidly to fight revolutionaries a power vacuum quickly emerged in the Russian occupied portions of the Balklans. Germany meanwhile would suffer from the 'Hungry Winter', German soldiers continued to fight until March when it became clear Russia had collapsed and many Germans were convinced continuing the war would mean true famine swept the land. Communists, Anarchists, and other revolutionaries rose up across Germany and the Germans rapidly abandoned the occupied AH Empire (though took with them any food they could, leading to famine, especially in Hungary). as Entente forces broke into Germany proper and pushed towards the Rhine Berlin negotiated a ceasefire. With millions of soldiers freed up (though most were quickly returned to civillian life) the Entente was free to deploy forces to the Russian far east and China.
With negotiations held in Versaille the Entente reluctantly admitted the Austro-Hungarian Empire was a lost cause, and instead attempted to push for ceasefires where possible. Between German withdrawal and Entente negotiations the Italians jumped into the frey, slogging it out with partisan forces and generally embarassing themselves due to a lack of experience. This poor performance saw the Entente force Italy to give significant territories to the Slavic Federation. Germany was let off surprisingly lightly, in part due to Entente exhaustion (and near bankruptcy), in part to avoid two nations collapsing to Communist radicals. While the world negotiated the Indian forces withdrew from the Ottoman Empire. Freed of outside observors the Ottomans unleashed a slew of deportations and massacres against Christian minorities in Anatolia, which prompted the Armenians to turn to Moscow with the Worker's Federation of Eurasia. Kurdish communists quickly joined in rebellion when a Eurasian army secured Russian Transcaucasia and began to move into Ottoman lands. Panicked by Muslims joining the rebellion the Ottomans launched a crackdown across the region which simply prompted Arab uprisings as well. Turning to the Entente for aid the Porte instead found her allies outraged by the mistreatment of Christian minorities, and it was only through Japanese mediation the Ottomans were merely placed on probation as an Entente member rather than expelled entirely.
By 1927 fighting has mostly wrapped up in the former Russian Empire and Ottoman Empire (though China remains chaotic actual fighting has mostly halted, with only occasional battles of above skirmishes). The global economy however has not recovered. The United States fell into a serious depression, having given numerous large loans to Germany and Russian they were left highly exposed to the Alliance's failure. Germany had chosen to let those loan debts fall as they attempted to pay reparations to the Entente forces, while Eurasia and other communist nations simply laughed at American bankers attempting to gain payments for loans the Tsar had taken. The United Kingdom and France both relied heavily on German payments to pay off their own debts (partially to Americans who had be open to do business with either side, though Brazil, Argentina, Mexico, Canada, Australia, and South Africa had all provided significant cash or gold). The rest of Europe would also suffer from economic stagnation, especially with paranoia over the 'Red Menace' rocking much of the continent.
Latin America however would see a period of prosperity (for the most part). With Europe owing them large sums of money the economies of the region saw a significant inflow of cash, and nations such as Mexico and Argentina saw massive modernisation schemes and economic growth. Venezuela would aslo see a relatively significant amount of growth, however a corrupt government led to the masses seeing themselves prosper far less than their Colombian neighbours (or Guyana neighbours who were admitted into Canada as a British cost saving measure) and soon saw socialist revolt break out. The US, busy with their own anti-communist purges, launched an intervention effort that has seen mixed results, but led to panic across Hispanic states. (Costa Rica turned to the US in a fairly unrelated coup to support American business interests, which despite the depression were still powerful).
Asia has seen fairly significant growth due to Japanese investment in exchange for easy market access for Japanese manufacturing. Some believe that the free trade zone has hurt Chinese, Korean, and Philippino industrialisation efforts, but most see it as a success for Europe and Latin America to mimic.
Canada has seen significant growth, and while life for the average citizen hasn't seen huge improvements there's enough land and mineral wealth to continue to need more workers, prompting immigration. The Australias have benefited similarly (though West Australia still refuses to join the Australian Federation, viewing provinces as insufficiently autonomous). South Africa meanwhile has had the awkward experience of attempting to incorperate the former Boer territories. Afrikaaners have been a headache for demands on language rights while also fighting Capetowns efforts at 'reasonable' treatment of racial minorities (a property requirement exists that only a very limited number of African and Indian merchants can fufill). South Africa is not the only region with racial tensions however.
As part of the 1915 Treaty of Versailles, the Racial Equality Act* incorperated into the 'Council of Nations'** has created a headache for colonial empires. Japan's request seemed like a good idea at the time, and was supported by France, Brazil, and others. France has since grumbled about it as her colonies have made noises for some sort of Imperial Federation, and Paris has appeased them by promising to either give them Dominion like powers or a federation once certain levels of 'development' have been met. London and Sydney had opposed the concept (much to Japan's outrage), however they've agreed to follow the laws. London insists all citizens of the empire are equal, and has officially begun an inquiry into doing . . . something to improve matters. Australia thought about rejecting the whole concept, however riots by Maori forced them to accept. Instead Australia has adopted an immigration policy to strongly select for family members of those already present (theoretically not race based).
Responding to the rights (theoretically) given in Council Nations a number of American cities saw race riots break out. Paranoid about communism, ethnic nationalism seen elsehwere, and the 'pro'-equality stance of the CoN American whites panicked and white mobs engaged in a number of massacres. Large numbers of blacks fled to Mexico or Canada, but most simply found themselves trapped. A press war broke out between American newspapers and Entente ones (who had wanted something to tar the Americans with after American attacks about war time misdeeds by the entente). In this era of increasing American isolationism and racial tension a small party known as the National Coalition Party has risen up in a number of segments of the South (which is generally richer and more populous than OTL, not as ravaged during the Civil War). The NCP have modeled themselves after a party in Italy which recently won parlimentary elections, a party which uses a symbol quite beloved by American iconographpy for it's name . . . the Fascists.
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*Japan proposed something similar at the OTL Paris Peace Conference, and it actually got majority support, but
somebody *cough*Woodrow Wilson*cough* decided it needed unanimous support . . .
**Consisting of most nations on earth. Notable exceptions are: The Worker's Federation, the USA, Costa Rica, Swabian Banat (for some reason), Switzerland, and 2 out of 3 Chinese governments.