Some terrible infectuous disease strikes mainland Europe in the '00s, and leads to the collapse of most industrial states. Britain and the USA both manage to survive by enforcing strict quarantines around their coasts.
Eventually, the disease dies down and the Britain and America ease their quarantines. In Europe, the royal families of Germany and Russia are all but eradicated, and so, the thrones of these nations are inherited by George V of Great Britain and Ireland.
The immediate post-epidemic era sees a period of unchallenged British expansionism, the Windsoric Realms are incorporated within the British Empire, French-held Africa is swiftly annexed and control over existing colonies is cemented.
The post-epidemic British Empire is run in a much more efficient way than its pre-epidemic form, people around the world are embued with a sense that the epidemic could have nearly destroyed civilisation, and that from now on, things must be done a lot better. Economic development is considered the main goal of the Empire.
Meanwhile, France, surrounded by Windsoric Realms to the north and east, and anarchy across the Pyrennese, feels increasingly insecure. In 1932, they sign a Treaty of Political Union with the United States. The USA has emerged from its period of epidemic-inspired isolationism, and feels itself destined to end the imperialism of the British. The arrival of the Americans onto European soil leads to the British re-evaluating France as a collapsed power, to a possible threat, and war is declared. British Germany succeeds in extending its border westward. Anti-British sentiment grows.
As the years go by, recently recovering nations feel compelled to either ally themselves to the USA, or the British Empire. Iberia, Italy, Turkey and Scandanavia join the American Bloc, while most of the rest of Eurasia lies under the political domination of London. By this time, a party calling themselves the New Whigs have acheived political monopoly within Britain, their ideology is secular, modernist and borderline-totalitarian in their intolerance of difference and opposition.
In 1953, the British Empire marches out of the Balkans and the Caucasus with the intention of conquering American-alligned Turkey. This leads to a huge war between Britain and the US. During this war the New Whigs abolish parliamentary democracy within Britain and scrap the remaining elements of civil rule in Germany and Russia and establish a federation of totalitarian one-party states, each owing supreme loyalty to Westminster.
While the War goes moderately well for America, both sides eventually begin to suffer from fatigue, and the war quickly comes to an end in 1956, when the British Empire drops two nuclear bombs on Paris and Marseille. This changes the tide, and a peace is hammered out at a conference just outside Toronto.
By 1958, the USA also developes nuclear weapons, and a delicate Balance of Terror begins to assert itself. Heavy fortifications are established along the borders of France, Scandanavia and Italy. These nations are officially subserviant to Washington, but retain a significant degree of autonomy and political freedom. However, social freedoms and welfare systems are much less established that in TTL, as such things are considered Whigish and anti-American.
In 1964, the Empire of Japan, the world's third biggest power, launches an attack against Pearl Harbour in Hawai'i. This leads to a war between Japan and the United States, the early campaign is characterised by island hopping, as both combatants push forward into each others islands, before being pushed back and so forth... Eventually, the USA acheives the upper hand, and comes into a position where it can invade the Home Islands. It is predicted by many that any invasion of Japan would become a complete quagmire, and that popular resistance would render their mission impossible. A few experts claim tactical nuclear strikes against Japan will be enough to amputate much of the resistance, but it is considered that such an action could perhaps lead to a retaliation from the British, and thus nuclear armageddon. And so, an old fashioned occupation of Japan begins, and as suspected, it is a total quagmire. The Emperor and his Government flee to Eastern China, where they are supported by the British, and where they, in turn, support the resistance in Japan.
The Japan War continues for twenty years as the Americans prove themselves again and again to be completely unable to curb a popular resistance. However, the USA does manage to support an underground resistance to the Japanese within Eastern China, and as they finally leave the Home Islands in 1985, they leave a full-fledged revolution against the Tenno in Mainland China. The Chinese overthrow their colonial masters and establish their own, free, pro-American state. The Japanese Imperialists flee back to Japan, but find that since a whole generation has grown up without their direct rulership that they are no longer welcomed, after another eight years of civil war, Japan establishes itself as the world's first, and only, socialist state in 1993.
In the meantime, things are not looking so settled for the British Empire. In Europe, the threat of American invasion is looking increasingly slim, and many, especially in Germany and Western Russia, begin to wonder why they should subject to Whigish domination to avoid it. In 1968, the German Nationalist Congress is established, with the aim of overthrowing British rule. In 1970, a similar organisation is set up in India, and in 1972, Russian nationalists join the fore. The London Whigs do their best to have these seccessionists sent to the Rehabilitation Camps in Siberia, but the seed of dissent is already deeply planted.
These whisperings eventually promote themselves to fully-fledged uprisings, and those in the USA who remain optimistic, (despite the events of Japan), begin to suspect that the British Empire is on the brink of collapse.
In 1987, German Nationalists seize control of Berlin and declare the Independence of the Third Reich of the German Nation. The Whigs in London have by this point come to realise that even within the regiments of loyalists in Russia and India, that they do not have the capacity to overthrow such revolts militarily, and as the months go on, the Germans notice that no tanks have rolled in, and so, the German state spreads and spreads. In 1990, Britain recognises Germany's independence.
This causes a domino effect, and soon, provinces of the British Empire around the world begin to declare their independence. In 1991, Canada declares independence, and later seeks to join the United States. In 1992, British South, British East, and British West Africa agree to form a federal African state. In the same year, the Federation of Belarus is formed from the former British territories of Finland, Poland, Ukrain and Western Russia - Transural Russia remains loyal to the British Empire.
India, however, remains Whigish heartland. The policies of responsible developement and mandated social harmony espoused by the New Whigs proved popular amongst the Indians, and when India finally did divorce itself from London in 1995, it did so under a radical Whigish coup, formed by individuals who felt that London was moving away from its own political ideals. The Indian Whigs continue to exercise complete political control to this day.
Meanwhile, in Europe, political plurality and democracy is on the march. Germany and Belarus both eventually join the international organisation borne of America and France's early union. The Atlantic Union guarantees free trade, free movement of goods and persons, small government... in short, a libertarian paradise.
The Whigs eventually collapse in Britain proper in 1997, and a period of political chaos and instability takes over, the British Empire, with its sole loyal province in Siberia, continues to chugg along, throwing abuse as the American World Order, whilst itself making little developement into democracy...
