Well, the Mongolia Empire was once the largest nation in the world, but due to the difficult to manage a Empire of that size, right after the death of Gankus Kan, it split into four Empire, Yan Empire that ruled China, Manchuria and Mongolia. Russia, Ottoman Empire and India. Yan Empire fall in less then 100 years, due to Mongols already struggle to control Southern China, because despise they seized the capital they, didn't take the states and each state governor, just become a King in its own right to keep fighting the Mongols, resulting in less then 100 years, the Mongols was over thrown and must retreat to Mongolia. the Ottoman Empire ended after WW1, due to the sudden surrendering of their ally Germany and Austria thus, not prepare for the sudden attack of the US, British and France alliance. India (original include, India, Pakistan and Afghanistan) split into civil war, by 18th century, afterwards, multiple warlords each control a small part of it, which give the British and French an opportunity to try conquer it. The British conquer India using the British East India company, a company that India originally hold 50% share. the Dutch which originally hold the other 50% share, sold their share to the British. The company backed by British government, ally with certain warlords against other warlords and eventually ruled, all of India by mid 18th century. After the British left, after WW2, it once again split into Pakistan, India and Afghanistan. Russia, declare independence from the Mongolia Empire, in 16th century, under the leadership of Peter the Great and his senior government officials, whom surprisingly, are also Mongols. After independence, within the next 300 years, Russia nobilities who are Mongols, would due to increase cross marriage, with Russian civilians who are Europeans and eventually became all white.
But eventually, it is possible for the entire world to united into a single nation, by transforming the UN, from an international alliance into a Republic and due to the advancement of modern technology, a global size Republic is for the first time in history possible to function. Who knows, the Republic of Earth might be possible, in the near future.