Other examples of nations merged together, with varying degrees of success: Yemen (North & South united in 1992, jury is still out on success), Italy (mix of regional small states and city-states until 1860s), Yugoslavia (1919-1991, RIP), Spain (Aragon and Castile until a personal union under Ferdinand and Isabella, late 15th cent.), Norway & Sweden (ca. 1814-1905, another relatively velvet divorce, like Czechs & Slovaks), Tanzania (union of Tanganyika and Zanzibar in 1964, some tension, but relatively successful), Vietnam (historically sometimes united sometimes divided, reunited by force after Vietnam War, lots of resentment in the south, including by many former Viet Cong types, but no signs that it is going to break up any time soon).
There are lots of examples of forcible annexation, some of which have proved long-lasting (some of the above examples border on it), and some not so much. East Timor and Western New Guinea were annexed by Indonesia--the former broke away, the latter is pretty firmly under Indonesian control despite resentment and resistance. Western Sahara was occupied by Morocco when Spain pulled out, and I think there is still a separatist movement there, but Morocco doesn't look like leaving soon. And there's Tibet and China, though China controlled Tibet loosely from the 18th to early 20th centuries and never relinquished their claim to it even when they didn't effectively control it (1913-50).
Anyway, dig enough, and you can find examples of all sorts of fusion and fission in the world of nation-states.