While Age of Empires III: Definitive Edition has been out since October 2020 and has been generally well-received, one major feature missing from the historical strategy is any form of co-op play - as of now that's no longer the case, as the game has just received an update to add co-op to the Historical Battles.
While Age of Empires III - both in its original form released in 2005 and the Definitive Edition that came out a year and a half ago - came with competitive multiplayer that remains popular even in its recent sequel, there hasn't been any sort of official campaign co-op available.
It's not quite the full campaign, but developer Tantalus has just updated Age of Empires III: Definitive Edition with co-op in the game's Historical Battles. All 12 battles based on historical events within AoE 3 are now playable with a partner, including all of the maps included with the DLC expansions - and according to the developer, only one player has to own the DLC for it to work.
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