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I am not sure 200-400 years are enough for that, but they might come close.
Expanding on Scimonster's answer I wanted to add two other examples of (partly) failed unifications:

Yugoslavia (pretty big cultural differences, languages, ethnic groups and religions) started out as a union of Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, Kosovo, Voyvodina and Slovenia; today however there is no more Yugoslavia, but instead a Serbia with an associated Montenegro (not 100% sure about that status quo right now)

Germany: Though there were the Holy Roman Empire and German Confederation before, true unification came only as late as with the founding of the German Empire in 1871, and even then there were dozens of little semi autonomous states (with which their inhabitants still identify, partly to this day) with a very dominant prussia. As you might know Germany was split after World War 2 and then reunited about 55 years later. A quarter of a century after that re-unification, you still hear people referencing to themselves or others as "Ossis" (inhabitants from the former GDR) and "Wessis" (from the former western parts of Germany); it is just a feeling, but that maybe decreasing right now; but over 140 years of the foundation of the German Empire there still are people primarily identifying as Bavarians, Saxons or what have you (some even as Prussians :D); but I am not under the impression that this unification should be called failed because of this, because most Germans will actually identify themselves as Germans (maybe even Europeans for that matter) when it comes to world politics, for cultural preferences (like food, music, dialects, etc.) this might be an entirely different thing.

I am not sure 200-400 years are enough for that, but they might come close.
Expanding on Scimonster's answer I wanted to add two other examples of (partly) failed unifications:

Yugoslavia (pretty big cultural differences, languages, ethnic groups and religions) started out as a union of Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, Kosovo, Voyvodina and Slovenia; today however there is no more Yugoslavia, but instead a Serbia with an associated Montenegro (not 100% sure about that status quo right now)

Germany: Though there were the Holy Roman Empire and German Confederation before, true unification came only as late as with the founding of the German Empire in 1871, and even then there were dozens of little semi autonomous states (with which their inhabitants still identify, partly to this day) with a very dominant prussia. As you might know Germany was split after World War 2 and then reunited about 55 years later. A quarter of a century after that re-unification, you still hear people referencing to themselves or others as "Ossis" (inhabitants from the former GDR) and "Wessis" (from the former western parts of Germany); it is just a feeling, but that maybe decreasing right now; but over 140 years of the foundation of the German Empire there still are people primarily identifying as Bavarians, Saxons or what have you (some even as Prussians :D); but I am not under the impression that this unification should be called failed because of this, because most Germans will actually identify themselves as Germans (maybe even Europeans for that matter) when it comes to world politics, for cultural preferences (like food, music, dialects, etc.) this might be an entirely different thing.

I am not sure 200-400 years are enough for that, but they might come close.
Expanding on Scimonster's answer I wanted to add two other examples of (partly) failed unifications:

Yugoslavia (pretty big cultural differences, languages, ethnic groups and religions) started out as a union of Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, Kosovo, Voyvodina and Slovenia; today however there is no more Yugoslavia, but instead a Serbia with an associated Montenegro (not 100% sure about that status quo right now)

Germany: Though there were the Holy Roman Empire and German Federation before, true unification came only as late as with the founding of the German Empire in 1871, and even then there were dozens of little semi autonomous states (with which their inhabitants still identify, partly to this day) with a very dominant prussia. As you might know Germany was split after World War 2 and then reunited about 55 years later. A quarter of a century after that re-unification, you still hear people referencing to themselves or others as "Ossis" (inhabitants from the former GDR) and "Wessis" (from the former western parts of Germany); it is just a feeling, but that maybe decreasing right now; but over 140 years of the foundation of the German Empire there still are people primarily identifying as Bavarians, Saxons or what have you (some even as Prussians :D); but I am not under the impression that this unification should be called failed because of this, because most Germans will actually identify themselves as Germans (maybe even Europeans for that matter) when it comes to world politics, for cultural preferences (like food, music, dialects, etc.) this might be an entirely different thing.

I am not sure 200-400 years are enough for that, but they might come close.
Expanding on Scimonster's answer I wanted to add two other examples of (partly) failed unifications:

Yugoslavia (pretty big cultural differences, languages, ethnic groups and religions) started out as a union of Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, Kosovo, Voyvodina and Slovenia; today however there is no more Yugoslavia, but instead a Serbia with an associated Montenegro (not 100% sure about that status quo right now)

Germany: Though there were the Holy Roman Empire and German Confederation before, true unification came only as late as with the founding of the German Empire in 1871, and even then there were dozens of little semi autonomous states (with which their inhabitants still identify, partly to this day) with a very dominant prussia. As you might know Germany was split after World War 2 and then reunited about 55 years later. A quarter of a century after that re-unification, you still hear people referencing to themselves or others as "Ossis" (inhabitants from the former GDR) and "Wessis" (from the former western parts of Germany); it is just a feeling, but that maybe decreasing right now; but over 140 years of the foundation of the German Empire there still are people primarily identifying as Bavarians, Saxons or what have you (some even as Prussians :D); but I am not under the impression that this unification should be called failed because of this, because most Germans will actually identify themselves as Germans (maybe even Europeans for that matter) when it comes to world politics, for cultural preferences (like food, music, dialects, etc.) this might be an entirely different thing.

I am not sure 200-400 years are enough for that, but they might come close.
Expanding on Scimonster's answer I wanted to add two other examples of (partly) failed unifications:

Yugoslavia (pretty big cultural differences, languages, ethnic groups and religions) started out as a union of Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, Kosovo, Voyvodina and Slovenia; today however there is no more Yugoslavia, but instead a Serbia with an associated Montenegro (not 100% sure about that status quo right now)

Germany: Though there were the Holy Roman Empire and German Federation before, true unification came only as late as with the founding of the German Empire in 1871, and even then there were dozens of little semi autonomous states (with which their inhabitants still identify, partly to this day) with a very dominant prussia. As you might know Germany was split after World War 2 and then reunited about 55 years later. A quarter of a century after that re-unification, you still hear people referencing to themselves or others as "Ossis" (inhabitants from the former GDR) and "Wessis" (from the former western parts of Germany); it is just a feeling, but that maybe decreasing right now; but over 140 years of the foundation of the German Empire there still are people primarily identifying as Bavarians, Saxons or what have you (some even as Prussians :D); but I am not under the impression that this unification should be called failed because of this, because most Germans will actually identify themselves as Germans (maybe even Europeans for that matter) when it comes to world politics, for cultural preferences (like food, music, dialects, etc.) this might be an entirely different thing.

I am not sure 200-400 years are enough for that, but they might come close.
Expanding on Scimonster's answer I wanted to add two other examples of (partly) failed unifications:

Yugoslavia (pretty big cultural differences, languages, ethnic groups and religions) started out as a union of Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, Kosovo, Voyvodina and Slovenia; today however there is no more Yugoslavia, but instead a Serbia with an associated Montenegro (not 100% sure about that status quo right now)

Germany: Though there were the Holy Roman Empire and German Federation before, true unification came only as late as with the founding of the German Empire in 1871, and even then there were dozens of little semi autonomous states (with which their inhabitants still identify, partly to this day) with a very dominant prussia. As you might know Germany was split after World War 2 and then reunited about 55 years later. A quarter of a century after that re-unification, you still hear people referencing to themselves or others as "Ossis" (inhabitants from the former GDR) and "Wessis" (from the former western parts of Germany); it is just a feeling, but that maybe decreasing right now; but over 140 years of the foundation of the German Empire there still are people primarily identifying as Bavarians, Saxons or what have you (some even as Prussians :D); but I am not under the impression that this unification should be called failed because of this, because most Germans will actually identify themselves as Germans (maybe even Europeans for that matter) when it comes to world politics, for cultural preferences (like food, music, dialects, etc.) this might be an entirely different thing.