Climate Change started the Third World War. Perhaps that's a gross oversimplification, but its the popular consensus of those who lived through that age. The world had just gotten used to the new coastlines with the loss of the South Greenland Ice Sheet, but in 2039 to 2041 the already tense geopolitical conditions were through into overdrive with the loss of Antarctica's coast-lying ice sheets, driving sea levels up to just under 30 meters above Pre-Anthropocene levels. All but a handful of the Earth's greatest cities were lost beneath the waves, and only the hearts of once great megalopolises survived. Billions of people were displaced, driving border protection policies by some countries while others aggressively tried to take advantage of potential additions to the workforce. Some of these people desperately tried to flee for the new off-Earth colonies, but Mars quickly reached pre-terraforming capacity, while the outer planets could barely handle a few million more citizens. Most were left on Earth to flee where they could. The most dramatic instances of this second wave of refugees/immigration to the developed world were seen in the low-lying countries that had already contributed substantially to the immigration boom in coastal Russia and North America. The newcomer was the former United Kingdom; with the storm of 2043, London and many major British cities were left underwater, with little hope of reversing the damage. With Australia's drought leaving the land virtually uninhabitable, and few other options available to them, a great British migration to the Americas occurred in late 2043 thorough much of the decade. With the Atlantic seaboard flooded, most settled in the American heartland like many refugees had for the the last decade. Those that did not settle in the United States made it to Newfoundland, and the newly independent first nations in the Arctic Circle to capitalize on the demand for skilled labor in the petroleum industry. Newfoundland went so far as to achieve independence from Canada proper in 2048, taking advantage of geographic isolation via Quebec. What remained of the United Kingdom quickly fractured. Scotland formalized its independence in 2044, Ireland reunified after the British military pulled out of Ulster to secure the British mainland, but to secure Britain itself the United States positioned 10,000 military and humanitarian operators in the former UK. By 2045 the British Parliament dissolved the monarchy to take direct control of its remaining lands to secure population interests, and the Republic of Britain became little more than an American protectorate. On the mainland Germany and France found themselves in a similar position with the Dutch and Belgians who had lost the majority of their nations to the Sea, and formally annexed these territories in 2049.
In the Pacific, facing a demographic disaster from an aging population and the loss of Tokyo to the Sea despite an immense government effort to construct a robust sea wall network, Japan began more aggressive campaigns into Pacific Russia, China and South East Asia to secure territory for its displaced population, which flooded by the millions into the Pacific Russian territories and Manchuria. Securing their economic and social claims meant a Naval buildup that conflicted directly with US naval interests, and despite formally cordial relations, the Japanese invested heavily into the construction of satellite tracking and ship destroying missile bases in the South Pacific. By middle of the 2040s Japan stood as America's chief rival in Earth Orbit and on the Moon, stationing civilian and military facilities at in Earth Orbit the Lagrange points and on the far side of the moon (though in this area they American presence will still dwarf that of Japan.) Turkey, facing much less loss of territory to the Sea, but rather a need for arable land from a dryer equator formalized its presence in the Caucasus with annexation, and a more aggressive presence in Southern Russia and the Ukraine to secure the Don and Volga grain belt. War along the Turkish axis defined the late 30s and much of the 40s as Turkey faced resistance groups to its power and conventional conflicts to secure its interests, culminating in the the Friday Revolution in Egypt, where Turkey deployed peacekeepers to secure regional interests following the flooding of Cairo, and effectively took control of the Suez Straight. With a position in Egypt, Turkey pushed its sphere west into North Africa, becoming the decisive power in Western Eurasia. Israel, already on friendly terms with the Turks and seeking an accommodation with the growing power, and Turkey not wanting to enter into a conflict with the small, but powerful enclave, entered into a mutual non-aggression pact in 2040. Control of the Suez Straight meant control over Arabia, despite endless conflict with Arab insurgents, and presented itself as a major threat to Iranian interests. As the dominant power over the oil and Natural Gas of Southern Russia to the Arabian Peninsula, Turkey found itself as a natural Ally of the Japanese, and established a formal Alliance in 2043, the Eurasian Trade and Security Coalition. While Formal alliances of this type were unusual in this age, the necessity of the global climate and refugee crisis drove these two powers to formalize their interests to secure mutual interests against the rising tide and against American interests. This event forced the United States to realize the new reality in Eurasia, as Turkey and Japan posed a credible risk to uniting the continent under a single power (though not a single state). With an economic depression in the Arab region, Turkey positioned itself as a neo-Islamist power, gathering support from pan-Turkics and Islamic fundamentalists. Facing mutual threats on two fronts, the United States increased support to India and China dramatically, going so far as to support Chinese nationalists over American and Taiwanese puppet regimes in Southern China for the sake of regional cohesion. By 2045, Poland, supported in the US effort to counter the Turks, brought Slovenia and Croatia into the Visegrad Group, despite a limited Turkish Presence in Former Bosnia. The Eurozone watched this developing crisis intensely, playing a game of wait and see, but finding itself on increasingly more amicable terms with the Turks, not wanting to see a United power on their Eastern Border again, and Germany once again made into a battlefield. At this juncture Germany began to a quiet effort to aid the Turks in every way short of war with Poland, by imposing greater influence on Danish and Croatian business interests to prevent Polish access to the Atlantic and therefore the United States.
The Americans pursued an extremely effective policy of containment against this new Coalition, arming the Poles, Chinese, Indians, and Koreans against the Turks and the Japanese, a policy that in turn drove the Japanese and Turks to increase their military readiness, and so on, driving the planet to war. With a halt in American trade of high tech goods to prevent any technology transfer, Japan and Turkey were left believing a full blockade could be next. With America supporting nationalists efforts in Arabia, Egypt, Russia, and the Ukraine, the Turks were convinced that war would be the next inevitable act by the Americans to cripple them. As such, war plans by the Turks and Japanese began to take shape.
The key American military threat no longer lied at Sea, but in Space. Since the Space Boom began in the late 2010s the United States had been quietly increasing its military presence in Earth Orbit and on the Moon, protective of valuable energy, mining, and manufacturing operations on Near Earth Asteroids and the lunar surface. During the 2030s the United States reached out to several contractors it had worked with on the commercial mining of lunar Helium-3 to develop increased infrastructure for commercial and eventually military ventures. Gradually the US abandoned the costly and ineffective policy of deploying overburdened infantrymen and petroleum burning vehicles to far corners of the planet to exert its power, to a system of rapid response, force projection technologies that could operate from mobile sea-based platforms and from US Soil directly. These systems still required a global command system, however and to that end the US began the construction of an extensive network of command and control satellites, all managed by three geostationary positioned Orbital Command Stations. Built in secret at the Tycho Shipyards on the moon, the Orbital Command Stations networked with unmanned spy satellites, refueling and repair facilities and orbital bombardment stations. No longer relying on vulnerable land based command centers, the United States could project its power with minimum effort and superb accuracy. Primary command and control moved to the Orbital Command Stations to limit the number of failure points, with space based systems seeming invulnerable to smaller powers. OCS Eisenhower was stationed over Uganda, OCS Kennedy over Papua New Guinea, and the main command station, OCS Reagan was stationed over Ecuador. These stations constituted the primary threat to the Turks and Japanese. Commanding fleets of hypersonic unmanned aircraft and coordinating Naval support around the planet, the Orbital Command Stations could impose devastating blockades on the Turks and Japanese. By 2050 war plans were well underway, and Japan new that to secure its regional interests it had to destroy the American command structure in space, and prevent any future launches by capturing the American manufacturing and military installations on the Moon. As the Japanese prepare for their attack, the Turks stage a crisis in the Balkans, pushing the Poles to the absolute edge of war, even allowing facilities in the Caucasus to be attacked by Polish drones. While the Geneva peace conference is being mediated by the Americans, Japan prepares to strike, hoping the Americans would rather accept a world with three great empires than risk its own empire in another World War.