The pursuit of unity is common to all nations, but in history, many nations have

The pursuit of unity is common to all nations, but in history, many nations have unified for once and then fell apart and never be able to unify again. Some examples are the Mongolian Empire, the Roman Empire, Napoleon, The Ottoman Empire, The Tsarist Russia (the later Soviet Union).
The questions that need to be answered: When were these nations established? When did they fall apart? Why they could not be reunified? The reasons of rise and fall. Compare the reasons of rise and fall between each nation, are there any similarities/ patterns/ common traits?
Please note that this is just a paragraph of a research paper, so all you need to do is answer the questions, make analysis and draw a conclusion. You do not need to write a full research paper with literature review, etc.
Please stay in touch and keep me updated on the progress. Thank you.