Short answers, pulled from the story.
A nation state is a centralized political organization ruling over a population within a territory that shares a common identity. This term describes a political entity where the state and the nation are broadly congruent unlike empires which comprise several territories and peoples established through conquest.
Most theories identify the nation-state as a 19th-century European phenomenon facilitated by state-mandated education and mass media. The Treaty of Westphalia in 1648 established the Westphalian system of states often called the modern system of states while early unified states emerged in Portugal and the Dutch Republic.
Nation states typically have more centralized and uniform public administration than their imperial predecessors and promote economic unity by abolishing internal customs and tolls. Language prohibitions were used to accelerate the adoption of national languages and the decline of minority languages while schools taught national history often in a propagandistic and mythologized version.
Israel was established in May 1948 as a homeland for the Jewish people with 75.7% of its population being Jews while Belgium serves as a classic example of a state that is not a nation state divided between Flemings and French-speaking populations. The Kingdom of the Netherlands presents an unusual example where one kingdom represents four distinct countries including Aruba Curaçao and Sint Maarten alongside the Netherlands.
When national boundaries that do not match ethnic boundaries are drawn ethnic tension massacres and even genocide sometimes occurred historically. Irredentist claims usually arise when an identifiable part of the national group lives across the border and these demands typically result in tensions and actual attempts at annexation are always considered a casus belli for war.