Which Pokémon game pair first introduced online Wi-Fi play to the main series?

Pokémon Diamond and Pearl first introduced online Wi-Fi play to the main Pokémon game series.

Released for the Nintendo DS, Diamond and Pearl added communication through the Nintendo Wi-Fi Connection. Players could trade Pokémon and battle across the internet, rather than relying only on a local link cable or nearby wireless connection. The games also included the Global Trade Station, which expanded trading between players in different regions.

The pair was first released in Japan on September 28, 2006. They formed the fourth generation of the main series and added 107 new Pokémon, bringing the National Pokédex total to 493 at the time.

Earlier games supported local communication, including link-cable trading and battling. That is why Ruby and Sapphire are sometimes incorrectly credited with online play. Diamond and Pearl were the main-series pair that brought internet connectivity through the Nintendo DS service.