Usually, we do not use the definite article, the, before names of countries but we use the before "United States of America".
Is there a specific reason for using the article before the country "United States of America"?
The is used in front of countries that have:
a general geo-political noun of place, e.g. republic, emirate, kingdom, state:
the United Arab Emirates, the United Kingdom.
On a related note, see this post on the capitalization of articles in names:
You only capitalize The when it is the first word in a title of book or a play. . . . In contrast, the word the is not normally capitalized in front of proper nouns that aren’t titles, like . . . the United States of America. . . .
Most country names are only a single word, or might include a geographical description (eg North Korea, South Korea).
The United States of America is plural. It means The States of America which are united. You cannot use States without The.
Other examples are:
The United Kingdom
The United Arab Emirates
The Netherlands (literally the Low Lands, also known as Holland)
Groups of islands also take The:
Using 'the' for a country name is a bit unpredictable.
As a guideline, country names that describe a physical geographic feature use 'the'. A group of islands, a river, or a mountain rage. For comparison, think of other geographical features such as the Sahara or the Amazon. Country names that describe a political features also use 'the'. This is usually a republic, or a political union - The United Kingdom or The United States of America.
The unpredictability comes when geographic or political features are unclear or forgotten. Lebanon is named after a mountain range, Sudan after a desert, Gambia after a river, and Ukraine = 'borderland', but none of those normally use a definite article.