Answer:
I'm not a 100% sure that it is correct but this is what I think,
A bald eagle and planes can represent symbols
join the Army Air Service can be a message
Americans who were old enough can be the audience
Explanation:
The bald symbolized the strength and freedom of America.
The " join the Army " can be like Uncle Sam wanting you to join the US military if you want a example
The Americans are the audience
