Asa was Alice Glick's brother. He died in World War I because he held onto a grenade for too long.
When Bart was working for Mrs. Glick (doing odd jobs around the house for her), he saw Asa's photo on a desk. Bart asked Mrs. Glick about the photo, and Mrs. Glick told him about her brother.
In a brief daydream sequence, Bart imagined what happened when Asa was killed. Asa, in Bart's daydream, was about to throw a grenade at the Germans and said, "This one's for you, Kaiser Bill. Special delivery from Uncle Sam and all the boys in in D company." Asa then went on to name several of his fellow soldiers, and while he was talking, the grenade exploded.