Complex sentence: Both migrations are dated to the second half of the 4-th century AD, and under the "disturbances" which caused them are considered to be the expansion of the Huns in the East-European steppes.
Split into simpler sentences: Both migrations are dated to the second half of the 4th century AD. The "disturbances" which caused them are believed to be the expansion of the Huns in the East-European steppes.