Complex sentence: Dubček's changes had broad support from the society, including the working class, but was seen by the Soviet leadership as a threat to their hegemony over other states of the Eastern Bloc and to the very safety of the Soviet Union.
Split into simpler sentences: Dubček's changes had broad support from the society, including the working class. However, it was still seen by the Soviet leadership as a threat to their hegemony over other states of the Eastern Bloc and to the very safety of the Soviet Union.