who were 'closer than friends closer than brothers'

The result of these shared experiences was a closeness unknown to all outsiders. Comrades are closer than friends, closer than brothers. Their relationships is different from that of lovers. Their trust in, and knowledge of, each other is total. They got to know each other’s life stories, what they did before they came into the Army, where and why they volunteered, what they liked to eat and drink, what their capabilities were. On a night march they could hear a cough and know who it was; on a night maneuver they would see someone sneaking through the woods and know who it was from his silhouette.
—  Band of Brothers by Stephen E. Ambrose