The American Defense Service Medal.

During a state of limited emergency from September 8th 1939 until December 7th 1941 a medal was issued to service personnel to boost their morale. The American defense service medal (ADSM) was proposed in February 1941 and officially established in June of the same year. The Army implemented the ADSM February 1942 with the Navy following in April.
The Army awarded the medal for 12 months service for the duration of the emergency but the Navy were less regulative and awarded the medal for active duty of at least 10 days.
The obverse showing a representation of Liberty with sword and shield standing on a four branched tree with each branch representing the four services, the reverse having the inscription “For service during the limited emergency proclaimed by the President on September 8 1939 or during the unlimited emergency proclaimed by the President on May 27 1941”.

The basic medal was awarded for service within the Continental United States (CONUS) but both army and navy also awarded clasps for service outside CONUS.

Foreign Service clasp: Awarded by the army for outside the Continental United States to personnel assigned to an aircraft, vessel or other duties.

Fleet clasp: Navy issue to those service personnel on a vessel or in an aircraft squadron in the Pacific, Atlantic or Asiatic fleet.

Base clasp: Issued by the Navy for on-shore service at bases and naval stations outside of CONUS which included Hawaii and Alaska.

Sea clasp: A Coast Guard issue to members who undertook regular sea patrols and who did not qualify for the Fleet clasp. A lot rarer than the other three above.

For those personnel who were on board vessels in the Atlantic prior to the US entering the war a ribbon was issued with an A device attached (Navy, USCG and USMC only).

A bronze service star could also be awarded for a subsequent award or involvement in an operation as shown in this set of Army ribbons (note the 3 battle stars on the EAME ribbon). The bronze service star could not be worn alongside the A device.
Not a rare medal to find, especially original issue although there are later restrikes available. All original issue medals had the slot loop suspender attached with later issues having the ½ inch crimp.
Resource used:
The Call of Duty, (R James Bender Publishing 1994) John E Sandberg & Roger James Bender.
American War Medals & Decorations. (Viking Press, New York. Leo Cooper Ltd, 1st UK Edition 1973) Evans E Kerrigan.