Complex sentence: The first Ba'athist branch in Tunisia, then under the unified Ba'ath Party, was established in 1955; however, the Ba'ath Party was outlawed under Zine El Abidine Ben Ali.
Split into simpler sentences: The first Ba'athist branch, then under the unified Ba'ath Party, was established in 1955. The Ba'ath Party was illegalised under Zine El Abidine Ben Ali.