Neustria was the other of the two “halves” of Francia (Austrasia being the other) that Charles Martel finally consolidated into something close to a cohesive whole by 721.
Neustria, meaning something like “new western lands,” as opposed to eastern Austrasia, was bordered on the north by the English channel, to the west by Brittany, to the south by river Loire, and to the east by much fuzzier boundaries. The kingdom of Burgundy was absorbed by the Neustrian kings early in the 7th century.
Neustria and Austrasia fought each other for most of the Merovingian centuries. Occasionally they would unite under a king for a time, but inevitably the Frankish inheritance rules broke the realms apart again. Finally in 687 Pippin of Herstal1.Pippin was of Austrasian descent, as were his descendents, Pepin and Charlemagne. defeated the Neustrian king at the battle of Tertry. After that the two kingdoms were ruled as one, while still subject to division in support of co-rulers. We can get some idea of how 7th century kings thought the various regions compared to each other from Fredegar, who says, “All the Austrasian magnates, the bishops and all the warriors of Sigebert, swore with hands raised that after Dagobert’s death Neustria and Burgundy united should belong to Clovis while Austrasia, which had the same population and extent of territory, should be entirely Sigebert’s.”2.Fredegar, Fourth Book, 76
After Charles Martel died in 741 the realm was divided between this three sons. Carloman and Pepin, both legitimate, received the bulk of the lands, with Carloman getting Austrasia and Neustria going to Pepin. Both sons also received outlying lands, but not, notably, Aquitaine. Grifo, son of Martel’s concubine Swanahild, was granted a much smaller area of twelve duchies. This led to much strife over the next few years.
When Pepin died in 768 he in turn divided the kingdom between his sons Carloman and Charles (Charlemagne). By this point the old distinctions were fading, and the term “Neustria” was not used in the division.
Years later Charlemagne began designating his heirs while he had many years to live, probably in order to prepare the kingdom for the inevitable division on his death. He revived the term Neustria and in 790 granted the land to his son Charles the Younger, then eighteen. Unfortunately the younger Charles died of a stroke in 811, and Neustria passed to new hands.