機動戦士Zガンダム外伝 ADVANCE OF Z 刻に抗いし者

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)