Ace Combat Zero: The Belkan War is the last PS2 Ace Combat title. Despite the name it's actually the sixth title in the series, although only loosely connected to prior titles.
This game takes place a decade prior to the events in Ace Combat 5. There are little references here and there that series vets can pick up on, but prior knowledge of the series isn't necessary, nor will it make you appreciate this game more. The general storyline is the escalation of fighting between two warring nations, and whether or not it will come to use of nuclear weapons. The player is a mercenary with a wingman who has more of a conscious than most mercenaries which leads to some interesting situations later.
Just like Ace Combat 5 broke from the mold of an Arcade-type 3D fighter simulation, this one continues that trend. As a mercenary you have certain targets in certain missions, but there are optional targets and some targets which are not advised to hit, like civilian housing. Depending on what you shoot, you fall into one of three categories of pilot, from most honorable, to most greedy.
The story is told through beautiful cutscenes and full motion video, with good, if sometimes over the top, voice acting from veteran voice actors. Radio chatter has become a staple of the series, and really keeps you involved in the action. The graphics are superb, but just like in prior games if you get too close to the ground it doesn't look that great.
Just like prior games, Ace Combat Zero doesn't exactly have the most realistic planes. There's one that shoots a laser, and all planes can carry dozens of missiles, unlike real life. However, the control of the planes is accurate, and the controls for the entire game are great as usual. Menus are easy to navigate, and the music is awesome. Unlike prior soundtracks this one has a touch of Spanish flare to go along with the techno/rock instrumentals that the series is known for.
The game is actually a bit shorter than Ace Combat 5, but has plenty of replay value. At the bare minimum you'll want to beat the game 3 times to explore the different story paths. The game no longer has the "arcade" mode like Ace Combat 5 did, but it's not missed too much.
Overall Ace Combat Zero is a great game just like all the Ace Combat games on the PS2 (and for that matter, the PS1). If you've never played the series, though, check out the other titles as well!