Which Pokémon games first introduced internet play through Nintendo Wi-Fi Connection?

Pokémon Diamond and Pearl first introduced internet play through Nintendo Wi-Fi Connection in the mainline Pokémon games.

Released for Nintendo DS, the fourth-generation titles enabled players to trade and battle online. They also supported voice chat and the Global Trade Station, which let players make trades with people around the world.

The games were first released in Japan on September 28, 2006, and later reached other regions in 2007. Their online features used Nintendo Wi-Fi Connection, a service that has since been discontinued.

Earlier Pokémon games supported local communication through cables or wireless hardware, but that is different from internet-based play. Pokémon Diamond and Pearl also added 107 new Pokémon and introduced the Sinnoh region.