>>34226
Pretty much.
In the final centuries of Han, the Empire managed to finally destroy their old rival the Xiongnu confederation. However, tribes of Xianbei and Wuhuan filled the power vacuum and start raiding the north, causing Han civilians abandon the northern frontier en masse.
Meanwhile, factional intriguing in the capital were undermining the authority of the central government, until the whole thing fell apart.
Cao Cao managed to restore some authority over the heartland, roughly half of the old empire by land, seven-tenths by population, but separatist regimes in the south and southwest remained defiant, which would lead to the division of Han into three claimant Empires after Cao Cao's death. However, though he was frustrated in the south he had great success in the north, breaking the power of Xiongnu, Xianbei, and Wuhuan tribes and keeping them from uniting. They were forced to settle into the northern territory abandoned by Han settlers, to keep them under watch.
For a while this worked, even after Cao Cao's death and the division of Han into three. Efforts were made to assimilate the tribal peoples, as tribal leaders were forced to send sons as political hostages to the capital, where they learned about Han culture. Some even began to take Han names.
The final reunification of the three states back into a unified Empire by Jin, however, had not coincided with a full restoration of Imperial authority. Cao Cao's heirs had failed to match his success, in part because both his son and grandson both died suddenly at tragically young ages, leading to a succession of child great-grandsons dominated by powerful regents. Eventually the Sima family gradually usurped power, and they were in part successful because they opposed the centralization of power that Cao Cao and his heirs had sought to enforce at the expense of regional powerful families. The Sima of Jin, therefore, were more like the leading family among many other leading families, and their authority was weaker than the Cao of Wei or Liu of Han. They were less able to keep watch on the north.
The diffusion of power was not limited to the Sima sharing power with other great clans, but the Emperor himself with the powerful princes of his family. When the weak Sima Zhong, who appears to have been actually retarded, was enthroned, immediately the Sima princes fought one another in a new civil war. And some of the northern princes called upon the settled barbarians of the north to join their armies.
But time as hostages had taught the barbarian leaders how to rule over Han people, and widespread disgust with the failure of Sima rule left many willing to serve barbarian regimes instead.