Complex sentence: The third battle at Vailele became the last significant engagement of the war, after, the Allies declared that so long as the Mataafans remain out of Apia, they would not take action against them.
Split into simpler sentences: The third battle at Vailele became the last significant engagement of the war. After it, the Allies declared that so long as the Mataafans stay out of Apia, they would not take action against them.