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Combined Arms games (Infantry, Aircraft, Armor) are pretty rare these days. The only other one I can think of is the Arma series, which are B2P and aren't really MMO on the scale of PS2.
Honestly, as infantry, there is nothing more fun than being involved in a huge PS2 tank battle. I was in the middle of one in one of the desert environments and felt like I was watching Rommel at work. Tanks were circling back and forth on the open terrain, and the infantry was leap-frogging from fox hole to fox hole forward.
Add in the dogfights overhead, and you really have something special.
Rithwis, Righteous Golem of Camelot | Skritha, Orc Archer of Tamriel | Bloodwod, Sawbones of Auraxis | Thrumdi, Blue Norn of Tyria | Gwidwod, Spider of the Ettenmoors | Gideon Slack, Hunter of Alsius