The story of
Ace Combat 5 centered around a small country named Belka manipulating two larger countries into war with each other, preparing to overrun both with it’s own army after they destroyed each other. In the background of this story was a fifteen-year-old war where Belka attempted to take over territory in a more conventional offensive.
Ace Combat Zero: The Belkan War centers around this conflict. In particular, it’s attack on its former territory of Ustio. Ustio’s army is not large enough on it’s own, so it has hired paid mercenaries to supplement it's air force. You are one of those mercenaries.
Ace Combat Zero is more arcade dog-fighting than flight simulator. You won’t be making realistic and detailed flight choices. Instead, the gameplay is mainly centered on taking down other planes as well as ground targets such as tanks, bunkers, and ships.
One of the new gameplay additions introduced in the game is a system that measures what kind of ace you are, a mercenary interested in money, a soldier bred for victory, or a man who lives by the code of the night. This system is made to sound far more involved and interesting than it actually is. Throughout the game, you will see targets marked as yellow. These are targets such as civilian houses, enemy tents, disabled (but not destroyed) planes, etc… These are targets you can earn money for destroying, but will not ever attack you directly. Put simply, destroy yellow targets and you will start leaning toward mercenary. Leave them alone and you will drift towards knight. Soldier sits in the middle of the two.
Another dynamic new aspect is more selection in choosing your missions. You will frequently get your choice of assignments during an operation, from destroying targets to escorting VIPs. Some of these mission choices can even play a part in small change in how the story plays out.