Definite Article Countries
Rule 7.2: Use the definite article with the names of countries that contain the words Kingdom, United, Union, Republic, State, or Commonwealth.
It never snows in the Kingdom of Thailand.
New York City is in the United States.
Mikhail Gorbachev was the last leader of the Soviet Union.
The same rule even applies to abbreviations of countries:
the United States → the U.S.
the Union of Soviet Socialist Republics → the U.S.S.R.
the United Arab Emirates → the U.A.E.
Extra! Use the definite article with the name of formal names of states, provinces, and cities.
the State of Florida
the Province of Ontario
the City of San Francisco