The infantry is the main land combat force and backbone of the Army

The infantry is the main land combat force and backbone of the Army. They are responsible for defending our country against any threat by land, as well as capturing, destroying and repelling enemy ground forces.

Job Duties

• Perform as a member of a fire team during drills and combat
  
• Aid in the mobilization of vehicles, troops and weaponry
  
• Assist in reconnaissance missions
  
• Process prisoners of war and captured documents
  
• Use, maintain and store combat weapons (e.g., rifles, machine guns, antitank mines, etc.)

http://www.goarmy.com/careers-and-jobs/browse-career-and-job-categories/combat/infantryman-11b.html