Which Pokémon games first introduced online trading and battling through Nintendo Wi-Fi Connection?

Pokémon Diamond and Pearl first introduced online trading and battling through Nintendo Wi-Fi Connection.

Released for the Nintendo DS in Japan in 2006, Diamond and Pearl expanded multiplayer beyond the local link cable. Players could connect through Nintendo Wi-Fi Connection to trade Pokémon and battle people remotely.

Earlier games supported local communication, and Pokémon Crystal offered limited online features in Japan through the Mobile System GB. Those services were not the same as the broad Nintendo Wi-Fi Connection functionality associated with Diamond and Pearl.

The games also introduced the Sinnoh region and the fourth generation of Pokémon. Their online systems became a major step toward the globally connected multiplayer features used in later titles.