The Expansion Period designates a war that occurred well before 8 ABY. It was waged between the Stromma species and the Quesoth insectoid race. Despite the Quesoth Soldiers employing relatively unsophisticated weapons, the Stromma were greatly impacted and eventually capitulated.
In 8 ABY, Grand Admiral Thrawn of the Empire of the Hand expressed his desire to examine the aged and incomplete records detailing the Expansion Period. This request was prompted by Stromma Council Liaison Nyama's reference to the conflict. Thrawn reasoned that understanding the ancient military strategies of the Quesoth, an insectoid species, could prove advantageous during the Empire of the Hand's attack against the Quesoth homeworld of Quesoth.
Timothy Zahn penned the Expansion Period's mention in his 2011 novella, Crisis of Faith. This novella was incorporated within Heir to the Empire: The 20th Anniversary Edition.