Indefinite and Definite Article with Military Institutions
Rule 8.10: Use the definite article with the words air force, army, navy, marines, military, fire department, and police when referring to them as an institution.
Quick! Call the fire department!
The president ordered the marines to strike.
Mark served in the military for 5 years.
Rule 8.11: Use the indefinite article with the words air force, army, navy, marines, military, fire department, and police when referring to their existence and/or characteristics.
Switzerland does not have a navy.
The United States has a strong military.
A professional army is always better than a conscript army.