Last December, Jose Ruiz called his mother, Juliana, to say a “big, big present” was on its way. A few minutes later, the doorbell rang. Ruiz was at the door, grinning.

“Here’s your present,” he said, wrapping his mother in a hug.

Ruiz, 28, of Brentwood, N.Y., was killed Aug. 15 by small-arms fire in Mosul. He was based at Fort Lewis. Ruiz attended the New York Institute of Technology and had worked as computer network engineer in New York but was inspired to join the military after the Sept. 11 terrorist attacks.

He is survived by his wife, Alexa, and their 9-month-old daughter, Liana, who met her father for the first and only time last Christmas.

“When he first met her, it was glory,” said his stepfather,

Eduardo King. “He never let her go.”

Ruiz was looking forward to coming home in six weeks, and had bought a house in...