Complex sentence: The Goths appear to have established a loose political hegemony over the existing tribes in the region, or at least to have played the leading role in a series of major invasions of the empire launched by a grand coalition of lower Danubian tribes from c.238 onwards.
Split into simpler sentences: The Goths appear to have established a loose political hegemony over the existing tribes in the region. Under the leadership of the Goths, a series of major invasions of the Roman empire were launched by a grand coalition of lower Danubian tribes from c.238 onwards.