Which department of the United States military was the first to be established and is the main line of ground defense for the United States?

a. Army

c. Air Force

b. Navy

d. Marines

The department of the United States military that was the first to be established and is the main line of ground defense for the United States was A. army. Today it is its largest branch for defense, and it has its roots in the Continental Army, established in 1775. Air Force, Navy, and Marines were created much later, and they are not used for ground defense - air force is in the air, and navy and marines on water.