In UK history, the union of the crowns in 1603 eventually led to the union of the nations in 1707.

Similarly, the union of the crowns of Aragon and Castile eventually led to the creation of the country of Spain.

However, despite Hanover and the United Kingdom having had the same monarch between 1714 and 1837, the nations never united. There are many other examples of countries having had the same monarch, but never effectively unifying - possibly one of the most recent being Norway which broke from its union with Sweden in 1905.

When was the last time than two countries effectively unified due to a dynastic union?

The last example I know of was (and this is highly disputable) would be the union of the crowns of Ireland and Great Britain in 1801.

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    There is a large list of personal unions in a wikipedia article which might give you some options to consider.
    – justCal
    Commented Aug 25, 2023 at 21:58
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    My guess would be Sweden and Norway, but as you mentioned, the two countries maintained significant internal autonomy and never really unified the way the UK did.
    – dan04
    Commented Aug 25, 2023 at 22:21
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    You also have the issue of places like Ghana, created on independence in 1957 as the union of the territories of Gold Coast, Ashanti, Northern Territories, and British Togoland which previously had separate forms of British colonial rule or protection or mandate, where Queen Elizabeth was theoretically the sovereign (or responsible for protection and the mandate) before and after the union.
    – Henry
    Commented Aug 26, 2023 at 9:25
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    Who thinks I'm pedantic, shoot me down and I suggest 'dynastic union' and 'personal union of thrones' simply don't match 'union of thrones'. 'Dynastic union' involves marriage, not mere inheritance. Here in the UK the union of the crowns in 1603 was due to the dying out of the English branch of a shared dynasty, not to any union. I also suggest 'personal union of thrones' was historically worth roughly the chat-room it was Posted in. Broadly, 'dynastic union' leaves a legacy but the very reason 'personal union…' is so called is that it tends to end with one person's death. Commented Aug 26, 2023 at 17:12
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    I'm not sure that there ever was an intent to unify Hanover and England. Certainly any impulse to do so didn't persist past George III
    – MCW
    Commented Aug 27, 2023 at 23:38

2 Answers 2


Saudi Arabia, 1932

Abdulaziz bin Abdul Rahman Al Saud, through a series of conquests, had expanded his dominion from the area around Riyadh to most of the Arabian peninsula by 1927, when he set up the separate kingdoms of Hejaz and Najd.

The British recognized King Abdulaziz's rule in exchange for his ending his expansionism, but this angered the Ikhwan who had been a large part of his army. They staged a rebellion, and began raiding into Jordan, Iraq, and Kuwait. Saud put this down with British assistance, but it took until 1931.

In the wake of the Ikhwan uprising, and similar to what the British did after the 1798 uprising in Ireland, King Abdulaziz consolidated the two kingdoms into one, proclaiming Saudi Arabia in 1932.

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    Thanks for your answer! I would have preferred if the kingdoms in question were not created and unified within the reign of a single monarch, but you have certainly given a correct answer to the question as written. Commented Aug 26, 2023 at 21:57
  • @NeilTarrant Well, Hejaz at least was a kingdom until Abdulaziz conquered it in 1923.
    – Spencer
    Commented Aug 27, 2023 at 0:02
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    +1 very interesting, but not sure that this fits the bill as a dynastic union. Whereas, if we speak about wars if conquest or political domination, one could give more modern examples not involving royals - like Hawaii becoming a US state or USSR integrating the Baltic countries (though this now has been reversed, the integration was substantial, as evidenced by the Russian-speaking "minorities".)
    – Roger V.
    Commented Aug 27, 2023 at 5:22
  • @RogerVadim Yet, this very specific action of setting up separate kingdoms and then merging them seems to fit the bill for me. Also, "thrones" implies monarchies.
    – Spencer
    Commented Aug 27, 2023 at 12:15
  • Union by conquest is a different proposition to the one envisaged by the question. It wasn't a personal union of separate monarchies than the 'led to' a subsequent political union, it was a state of conquest in which the conquered kingdoms had no say, arising from it being convenient to the conqueror to create a single new state.
    – fred2
    Commented Aug 29, 2023 at 20:19

The modern state of Romania appeared on the map in the 19th century as a result of a "personal union" of the principalities of Moldavia and Wallachia under the prince Alexandru Ioan Cuza.

These two countries had an elective monarchic system of Byzantine origin. Under Ottoman suzerainty, their princes were initially, at least theoretically, "elected" by the local elites or even by popular acclaim, and then confirmed by the Sultan, but by the 18th century the formalities were reversed: the Ottoman court selected the prince, who was then formally "acclaimed" by the locals.

After the Napoleonic wars, the two countries fell under Russian influence and even military occupation, the Ottoman rule becoming mostly nominal, limited but still important in relation with the nomination of the price. Administrative union was promoted by Russia through the establishing in the two countries of the same set of laws (a proto-constitution, Regulamentul Organic) in view of a future annexation.

But Russia's plans of annexing the two countries fell short with the Crimean War.

I have found a clear but exhaustive presentation of the context of this event in Daniel Chirot, Social Change in a Peripheral Society, 1976, p.112-117:

The Crimean War was fought over a number of issues that revolved around Great Britain's desire to curb Russia's threat to the British Empire, particularly to its "lifeline" running through the Eastern Mediterranean. The fate of the Danubian principalities was one of the important points of the dispute. Romanian grain exports were becoming increasingly important to Britain, and Wallachian and Moldavian markets seemed promising to British industry. Both the French and the British wanted more stable, cooperative regimes in the principalities. But the most important issue was that both countries wanted to limit Russian power on the Danube.

... issues had to be settled with respect to the principalities... should they be united into a single Romanian state? (It was recognized that they shared a common language and rather similar institutions and historical traditions.)

... These questions, it was decided, were to be settled by the seven interested powers—Great Britain, France, Austria, Turkey, Russia, Sardinia, and Prussia. The first five of the seven were the most important parties in the discussions to decide these issues. Sardinia followed France's lead on every question in order to maintain its alliance with Napoleon III against Austria; and Prussia was essentially neutral in the discussions, as well as being more removed from the issues in which it had no serious interest.

...Great Britain was most interested in the international issues and felt that no substantial internal changes were necessary in the principalities. Rather, the British hoped that the principalities could simply revert to tighter Ottoman control. Austria was more interested than Britain in the domestic problems of the principalities because it ruled a large Romanian population in Transylvania and Bukovina, and because once again it hoped to expand its economic and political influence down the Danube. But Austria had not participated in the Crimean War against Russia, choosing instead to remain neutral, and the only way it could have gained its Danubian ends would have been to give up northern Italy in return. This latter solution, proposed by Napoleon III, was rejected by Austria.

Russia had lost the war and therefore resigned itself to losing its hegemony over the principalities. In an astonishing reversal of its policy, Russia began to support the nationalist liberals in Romania on the question of national unification. This was for the double purpose of annoying the Austrians and of gaining popular support within the principalities. A further consideration in Russia's new position was its desire to gain a friend among the victorious Crimean powers, and Napoleon III had a vague ideological commitment to the Romanian nationalists. The Ottoman Empire simply wanted to regain some of its lost influence in the principalities in order to block future Russian expansion toward Constantinople.

...A related issue... was whether or not the principalities should be ruled by native or foreign (i.e., Western European) princes. The diplomatic haggling that went on at the various post-Crimean War conferences would have been comical, if the fates of millions of Eastern Europeans had not been decided by considerations of the balance of power between the major powers...

...The main issues, both international and internal, were still not settled by 1858, so another conference was held in that year at Paris. The principalities were placed under a collective guarantee that was to be enforced by the seven powers. Turkish suzerainty was affirmed, but defined very narrowly: The Sultan was given the right to "invest though not reject" the princes of the principalities (though the meaning of this phrase was not particularly clear)...princes were to be chosen for life by assemblies in each of the principalities. Wallachia and Moldavia were declared autonomous in their internal affairs. Some measure of government contact between the two principalities was provided for, but full union was ruled out...


In 1859, the assemblies of Wallachia and Moldavia elected princes. Much to the surprise of the seven powers, the boieri [nobles, big land owners], secure in their control of the situation, had been won over to the nationalist cause, and both assemblies elected the same prince, Ioan Cuza. By this act the two principalities were united, and in 1861 they were formally joined into a new Romanian state (which was, however, still theoretically subject to the Ottoman Empire). Faced by this Romanian action which was supported by Napoleon III, the powers accepted it.

This event anticipates up to a point the one discussed in the accepted answer (on Ibn Saud), which is more recent, but this has the advantage of referring to previously existent states, not newly created by the unifying ruler.

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