He attained the rank of major general, and was most notable for his service as commander of the Americal Division and Superintendent of the United States Military Academy.

[3] During the war he also completed his Infantry Officer Advanced Course and graduated from the United States Army Command and General Staff College.

[3] During the Korean War Koster returned to Asia, serving with both Operations and Training (G-3) and G-2 staff sections of the Far East Command and the Eighth United States Army.

[3] In April 1966, he was assigned as director of the Plans and Programs Division in the Office of the Army's Assistant Chief of Staff for Force Development.

[3] By 1967 he had attained the rank of Major General, and at the height of the Vietnam War was assigned to command Task Force Oregon.

[4] While no official count was made, soldiers and investigators later estimated that 350 to 500 women, children and old men were killed with grenades, rifles, bayonets, and machine guns; some were burned to death in their huts.

[4] To make matters worse, these reports were never sent to headquarters, as military protocol required, until an Americal veteran named Ron Ridenhour triggered a secret high-level investigation by sending a three-page letter detailing the evidence he had uncovered about the massacre to the Pentagon, the president, and members of Congress in March 1969.

[4] He was subsequently censured in writing, stripped of a Distinguished Service Medal and demoted to brigadier general for failing to conduct an adequate investigation.

[7] Following his demotion, Koster was reassigned as deputy commander of Maryland's Aberdeen Proving Ground, in charge of Army weapons testing.

[1] After his retirement, Koster worked for 12 years as an executive vice president for the power transmission division of Koppers and Hanson Industries in Baltimore.