Kidou Senshi Z Gundam Gaiden: Advance of Z - Toki ni Aragaishi Mono

In the year UC 0079, during the One Year War of the first Gundam series and eight years before the story in Zeta Gundam, a boy named Van Asiliaino and his friends attempt to flee the war, and their fates change when they meet a certain soldier.

(Source: Gundam Wiki)