Small skirmishes and border conflicts were fought between the New Republic and Chiss for several weeks before the attack on Gala. The border disputes were not officially recognized as war so the Battle of Gala was not an act of war but an act of terror.
After the battle, the New Republic declared war against the Empire and Chiss. Soon after, they discovered the Venen Empire was involved with the attack and the Senate declared war against them. The Senate called for the Jedi to join them but the Council initially decided against it. Polo Hudorra felt there was enough evidence for a legitimate reason to go to war and he overturned the Council's decision.
The Battle saw that first time that the Dark Troopers would fight in real combat. It was the first combat mission for many Chiss Marines, too. Amongst the Dark Troopers and Marines was those of the Stars Squadron and its newly appointed leader, Qurnin Son. Son, appointed shortly after the battle's beginning by the Unknown Sith Lord, would go on to become one of the most famous Dark Troopers, going on to become the first Dark Trooper to become a General in the Sith Imperial Army.