GUILDERLAND, (New York) (REUTERS) - The father of Major-General Harold J. Greene on Wednesday described his son, killed in an Afghanistan insider attack a day earlier, as "unique to the military," a popular kid whose intellect led to his military success.

In an interview at his home in upstate New York, Harold F. Greene told the story of a boy who made a city out of a sand dune, was so smart that Guilderland High School allowed him to skip his senior year, and, as an adult, shaped the weapons used by the military.

"(He) was a kid I could be proud of," his father said. "It's our country that lost, really."

The eldest son of Harold F. Greene and his wife Eva, who died in February 2013, found an "accidental" route to his military career, his father said.

During his freshman year at Rensselaer Polytechnic Institute (RPI) in Troy, New York, he enrolled in the ROTC programme to fulfil a physical education requirement required by the college.

His ROTC performance earned him a full academic scholarship, a boon for the family with two other boys about to enter college as well.

It also gave his family and the military a preview of what Greene would become, as he rose to Cadet Commander and earned the rank of Second Lieutenant upon graduating from RPI - with both bachelor's and master's degrees - and began his military career.

Greene visited most of the US combat zones during his military career, but was never put on the front lines. His role was more technical, dealing with the processes of developing and implementing weaponry.

His father said that the drone programme is a byproduct of his son's early career.

"He was unique to the military," his father said. "He was performing a function that took in everything from research to development and he helped develop weapons systems that really help save a lot of lives in the field."

As a captain, he married his wife, then-Captain Sue Myers. Myers, now a retired colonel, who is a professor at the US Army War College in Carlisle, Pennsylvania, where the family has a home. The couple have two children, Amelia and Matthew, who is a West Point graduate and a first lieutenant in the Army.

Greene had received his second star as a general and was eligible to retire, but he opted to continue his career.

The last time the elder Greene saw his oldest son was December, but it was just a coincidence that it was around the winter holidays. Major-General Greene made it a point to visit his family whenever he returned from a tour.

He was shot and killed in Afghanistan on Tuesday. His body will arrive at Dover Air Force Base early on Thursday and he will be buried at Arlington National Cemetery.