Complex sentence: We see in flashbacks that Logan stopped writing to her not long after he went off to war, and has not seen her since he spent one day with her four years ago, the day he came back from World War II as a decorated soldier.
Split into simpler sentences: We see in flashbacks that Logan stopped writing to her not long after he went off to war. He has not seen her since he spent one day with her four years ago, the day he came back from World War II as a decorated soldier.