Complex sentence: Piracy in the 21st century is most often associated with the actions of Somali pirates, since 2005, and the second phase of the Somali Civil War, acts of piracy in the region have risen dramatically.
Split into simpler sentences: Piracy in the 21st century is most often associated with the actions of Somali pirates, especially since 2005. During the second phase of the Somali Civil War, acts of piracy in the region have risen dramatically.