Which core Pokémon games first introduced online trading and battling?

Pokémon Diamond and Pearl first introduced online trading and battling in the core Pokémon games.

Released for Nintendo DS in 2006 in Japan and in 2007 in many other markets, the games used the Nintendo Wi-Fi Connection. Players could connect through the Global Trade Station to exchange Pokémon and could battle friends online.

Earlier games supported local communication through cables, infrared connections or wireless features, but Diamond and Pearl made internet-based interaction a central part of the main series. The Sinnoh games therefore marked a major shift in how players built teams and competed.

The Nintendo Wi-Fi Connection service was later discontinued, so the original online functions no longer operate as they did at launch. Later games replaced that infrastructure with newer Nintendo network services and their own online systems.