Upon graduation, David was commissioned Second Lieutenant with an assignment in the Corps of Engineers. He spent the next eight years on active duty and reserve status.
Eller was initially stationed at Ft. Belvoir, Virginia. While there, he was assigned to the staff of the Commanding General for the Headquarters of the Corps of Engineers, working in the G-2 Intelligence area.
As the threat of nuclear war with the Soviet Union increased and the Berlin Crisis occurred in 1960, David was relocated to Germany and became the commanding Officer of the 541st Combat Engineer company, 11th Engineer Group, Seventh Army, based in Schwetzingen, Germany. The majority of his tour of duty in Germany was spent in combat ready condition at the East German / Soviet Union border.
Upon completion of his military service, David remained in Germany and accepted a position as a chemical engineer for Warren & Co, GmbH, a German engineering firm located in Frankfurt, which specialized in the engineering design and construction of petroleum refineries and petrochemical plants. While employed there, he served in several management positions before being promoted to the position of European Manager two years later. The promotion made him responsible for the company’s engineering and construction operations in approximately twenty Western and Eastern European countries.