Paul W. answered 02/03/22
Dedicated to Achieving Student Success in History, Government, Culture
IN LIFE, THE ONLY GUARANTEES ARE DEATH AND TAXES. Human society is diverse - different genders, sexual identities, different 'races' (a commonly used term that is incorrect - we are all part of one race, the human race), different ethnic identities, different religious Faiths (or lack of a Faith), different socio-economic classes, different lifestyles dictated by location (urban versus rural, for instance), different occupations, etc.
With all of this in mind, it should be easy to understand the difficulties facing a national leader who seeks to united the population of a country behind him / her. It's safe to say that no national leader in all of history has united 100% of the population of a nation under their leadership. Even in a totalitarian state, such as the Soviet Union, in which people were subjected from birth to official propaganda that promoted unquestioning obedience to a leader, there will be a certain percentage of the population that is opposed to the nation's leader (moreover, what that percentage was is impossible to determine, because, in a totalitarian state, people cannot openly express their true opinions).
The best any national leader can hope for is to win the support of the majority of the people in the nation in question. Indeed, this is the basis for a democratic form of government. Without the votes from a majority of the population, an individual cannot win the right to serve as a nation's leader (though, in the United States, which claims to be a democracy, recently two Republican candidates - George W. Bush and Donald Trump - became president, event though more people cast votes for the Democratic candidates. This was due to the archaic nature of the system of 'democracy' in the U.S.).
A national leader may be able to gain the support of the majority of a nation's population by appealing to something that is commonly shared by a majority of the population. This can include a specific ethnic / cultural identity, a specific religious Faith, a specific historical event, etc... Nor are these mutually exclusive, that is, they may be used in combination.
Of course, a national leader can appeal to the people of the country he / she is leading on the basis of their shared national identity. Nationalism is a relatively recent concept in human history, originating with the ideas that motivated both the American and French Revolutions in the late-18th century. Essentially, the concept of 'nationalism' argues that a country belongs, not to a king or emperor, but to all of the people who live in it. These people are known as 'citizens' because they have rights that, ideally, cannot be violated, including the right to have a say in what type of government they want to be ruled by and who should lead that government. Citizens, in turn, are personally responsible for the defense of 'their' country. In other words, they owe their loyalty to their country. It's this loyalty that a national leader can call upon to theoretically unite the people of the country they lead.
The problem is that loyalty to one's country can conflict with a person's other loyalties, such as their loyalty to their family (if your brother is an enemy spy, do you turn him in?), loyalty to one's local community (the building of a dam will provide electrical power for the nation, but it will result in the loss of farmland for your community), loyalty to a religious Faith (your country is at war and needs citizens to serve as soldiers, but your religious Faith teaches that killing is a sin), etc...
While it's not impossible for a national leader to 'unite' at least a majority of a country's people, for the reasons listed above, it's certainly not easy.