The Declarative TheoryThe Declarative Theory of defining a country follows the belief that a country declares itself a country or is declared a country once it has achieved certain criteria. The Montevideo Convention of 1933 (link to full text here) (link to explanation here) was signed by 19 countries. Although this is by no means a majority of countries, the guidelines set forth in the convention have served (somewhat) as de facto requirements to become a nation.
These requirements are:
“a ) a permanent population;
b ) a defined territory;
c ) government; and
d) capacity to enter into relations with the other states.”
These requirements seem to make sense; they allow a nation to exist physically and politically. Despite many nations having signed this document, it is by no means a list of necessities set in stone. However, these four points are often sought by prospective countries as goals toward statehood.
Constitutive TheoryMany nations, particularly those who are members of the United Nations or seek membership there, believe that recognition by current states is necessary before a nation may be considered such. Many of these nations wish to reserve the privilege of nationhood to those nations which gain recognition by other nations, often as political allies. This system would mean that for any nation to become a nation, it would need to be recognized by other nations (though by how many other nations is uncertain). This system would also give power to those nations which already exist over those nations which are young or may not yet hold the title of nation. The real reason for withholding nationhood may be more so because with each new nation, the influence of the old ones lessens, and seemingly no nation would actively reduce its influence.
Luckily for aspiring nations however, there is no universal agreement on how to become a nation. There exists two main theories: the declarative theory, which follows the guidelines of the Montevideo Convention, and the constitutive theory which follows the idea that recognition is most important for nationhood.
The fact that there is no agreement means that for new nations (perhaps those that cannot gain all of the requirements enumerated in the declarative theory or the recognition necessary for the constitutive theory) becoming a nation is all in their own beliefs of themselves. That is, a nation becomes a nation when it believes it is a nation, not because of meeting criteria or because others believe it is a nation. Ideally, this means that anyone could create a nation, but doing so may lessen the intended impact of creating a nation. Even so, the definition of a nation is not concrete and is left up to the individual (or collection of individuals) of that country.
What seems most necessary for the creation of a nation is at least one individual (though more, while not necessary, may add to legitimacy and influence). This individual(s) needs to be willing to further their cause as a nation, whatever that may be. Any other criteria are extra but are generally necessary for the creation of a nation which intends to encompass a large group(s) of people. A leader tends to be beneficial here as well. But as previously mentioned, the final determination of what constitutes a nation resides with the creators of that nation and its citizens, not with anyone else.
For a further in depth analysis of declarative vs. constitutive theories of statehood, click (here).