If we're going to talk about "nation building," it would be extremely helpful to define the word "nation." According to dictionary.com, a nation is "a large body of people, associated with a particular territory, that is sufficiently conscious of its unity to seek or to possess a government peculiarly its own."
The American Heritage Dictionary defines "nation" as "A relatively large group of people organized under a single, usually independent government; a country."
The word came from an Old French word nacion which came from a Latin word which meant "to be born." Although "nation" originally meant a large group of people of the same race, the political meaning of shared ideas and shared government has gradually replaced the idea of shared genetic heritage, although "nation" is still used in that sense when referring to native Americans. This idea is retained in a secondary definition, "an aggregation of persons of the same ethnic family, often speaking the same language or cognate languages."
Even if all the people are from pretty much the same racial background, they may or may not share a common language. China is a nation in the sense of shared ethnic background even though there are many different Chinese dialects which are not mutually understandable.
The political meaning implies that all of the citizens of a nation live in a more or less contiguous area, but we speak of the "Jewish nation" even though Jews are scattered all over the world.
The "Jewish nation" is exceptional in that Jews live under many different governments in many different places, but their example shows that a strong enough sense of unity can define a nation, no matter how scattered its members. It also applies to the Kurds, who live in a contiguous area at the intersection of Iraq, Iran, and Turkey. Although Kurds share ethnic background and a strong dislike of the governments under which they live, none of the nations where Kurds live is interested in setting aside territory for an independent Kurdistan.
Kurdish dreams of a nation with an independent government to go along with their shared culture and race may not happen any time soon. However, the primary definition of "nation" says, "sufficiently conscious of its unity to seek or to possess a government peculiarly its own." Most Kurds seek a government of their own, so they meet the definition of "nation."
Given that a nation need not have a common territory, or language, or ethnicity, or even a common government, the necessary element is "sufficiently conscious of its unity." A large enough group of people are a nation if they are sufficiently conscious of their nationhood.
There have been political entities which were thought to be nations which turned out not to be unified enough. Marshal Tito ruled Yugoslavia with an iron hand for 45 years after WW II. His secret police kept the country together, but when his government was weakened by his death, Yugoslavia split into so many different countries that it's hard to keep track of them all. To save time, we often refer to them collectively as "former Yugoslavia." Some of the fragments appear to have enough unity to become nations, but only time will tell.
Although it occupied a contiguous area with a unified government, Yugoslavia did not have sufficient unity to remain a nation when Tito died. We see the same effect in large corporations. When Harold Geneen retired, ITT, the conglomerate which he built, suddenly became less profitable and fragmented.
India was not a nation when the British left; it split immediately into Pakistan and India. Pakistan later split as Bangladesh declared independence. Ireland had to be split to keep the Protestants and Catholics apart. The United States split into two nations in 1861. The north did not accept the partition and killed enough southerners that the survivors were willing to surrender their independence and rejoin the union.
The United States has been a nation since winning the War of Independence. At the beginning, it had a shared language, a reasonably widely shared belief in Christianity, and shared ethnicity. The consensus regarding Christianity was powerful enough that an early Bible was printed by the Congress of the United States for use in schools, but these unifying factors could not overcome the disagreements that led to the Civil War.
Since the Civil War established unity by force of arms, American ethnic unity has been diluted by immigrants from many countries; linguistic unity has been undermined by the movement to require the use of other languages in schools and in public areas; and religious unity has been undermined as religious fervor has dissipated.
What's left? About the only common element of American culture is a desire to get rich. Will greed be enough to hold America together? Time will tell.
What does Chinese history have to teach America that Joe Biden doesn't know?