June 1812:
The coountries surrounding
Luxembourg
voluntarily start joining it, one by one,
But in June 1812, isn't Luxembourg just a 'department' or two within the French Empire?
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1860s: When the British Empire grants 'dominion' status to Canada it also sets up an Imperial Parliament -- to handle international affairs, and defence -- in which the UK, Canada (and, later on, other dominions) and self-governing colonies are all represented. As time goes by this system expands to cover not only lands that had actually been under British rule but also various others that see membership as an advantage for reasons of mutual defence and/or 'imperial preference' in trade: Our old ally Portugal joins shortly after becoming a republic in 1910, for example.
Nations that lose wars in which the British are on the winning side get offered associate membership as a part of the 'reconstruction' process, as do new nations that arise as a result of other empires fragmenting during those wars, and in many cases those countries eventually apply for transition to full membership instead. As the 'Commonwealth' (as it is now called) continues to expand, some nations are drawn into joining simply because their neighbours have already done so and getting left out therefore seems inadvisable. Eventually the momentum becomes unstoppable...
1st June, 2012: The last seven formerly-separate nations on Earth (USA, Alaskan Free State, Switzerland, Vatican City, Bolivia, Paraguay, and Pakhtunistan) all join the Commonwealth -- by treaties -- on a single day.