Complex sentence: It saw the Oilers, Whalers, Nordiques, and Jets join the NHL for the 1979--80 NHL season, but the NHL insisted on treating the deal as an expansion rather than a merger.
Split into simpler sentences: It saw the Oilers, Whalers, Nordiques and Jets join the NHL for the 1979--80 NHL season. However, the NHL insisted on treating the deal as an expansion rather than a merger.