Complex sentence: The ISO and CPNZ did in the end fuse to form the Socialist Workers Organization albeit a majority of the former ISO soon left as a result of what were seen as Stalinist practices on the part of the former CPNZ leadership.
Split into simpler sentences: The ISO and CPNZ did, in the end, fuse to form the Socialist Workers Organization. However, a majority of the former ISO soon left as a result of what were seen as Stalinist practices on the part of the former CPNZ leadership.