The word army is both a common and proper noun. Determining when it should be capitalized largely depends upon the meaning intended and the context in which the word is being used.
According to Merriam Webster the word army can mean any one of the following:
- a large group of soldiers organized to fight battles on land
- the part of a country’s military force that includes soldiers who are trained to fight on land
- a large number of people or things involved in some sort of activity together
In the first case of the definition, a large group of soldiers organized to fight battles on land, army is not capitalized since it is not referring to a specific organization and therefore not a proper noun.
Here are some examples:
- The two armies engaged in hand to hand combat against each other.
- The army’s campsite was located on the outskirts of the city.
- We wrote letters to the soldiers in the army thanking them for their service.
In the second case of the definition, when the army refers to the United State’s military, the word army is always capitalized because it refers to the proper name of an organization.
Here are some examples:
- Under the constitution, the President is the commander and chief of the United States Army.
- My cousin is currently serving in the United States Army.
- He was undecided as to whether he wanted to join the Army or Navy.
In the last sentence, the word army is still capitalized even though the words United States are missing from the sentence. This is because in this example army directly refers to the United States Army.
However, when writing about the military of a foreign country, or using the term in a generic fashion, the word army is not capitalized.
Here are some examples:
- The Russian army failed in their attempt to bring down the foreign government.
- China’s army has continued to grow in strength and numbers.
- The American army passed out blankets and supplies to the people of the war-torn nation.
In the third case of the definition, when army refers to a large group of people involved in the same activity, army is a common noun and therefore not capitalized.
Here are some examples:
- I watched an army of ants crawl through a crack in the door.
- An army of seagulls descended upon the beach searching for food.
- When disaster struck, an army of volunteers flew to region to assist in the cleanup operations.