Battlefront II is fundamentally similar to its predecessor, albeit with the addition of new gameplay mechanics. The general objective in most missions is to eliminate the enemy faction. Like Star Wars Battlefront, the game is split into two eras, the Clone Wars and the Galactic Civil War. Players have the ability to choose between six classes during gameplay. Four class types are common to all factions; infantry, heavy weapon, sniper and engineer. In addition to the four standard classes, each faction has two unique classes which unlock by scoring a predetermined number of points. For the Rebellion faction, the Bothan Spy and the Wookie; for the Empire, The Officer and The Dark Trooper; for the Republic, the Commander and the Jet Trooper; and for the CIS, the Magnaguard and the Droideka. Battlefront II also features heroes - a special class that allows the player to control iconic characters from the Star Wars universe. Heroes serve as a bonus to players, and are unlocked by meeting certain criteria, such as earning a certain number of points. Differences between Battlefront II and its predecessor include the ability to sprint and the ability to roll forwards.
Whereas Star Wars: Battlefront's campaign featured missions dependent on the chosen faction, Battlefront II contains only one campaign, called Rise of the Empire, which is found in every version of the game except the PSP. This set of missions is presented as the narration of a veteran of the 501st Legion, known as Vader's Fist, beginning with an attack on the planet Mygeeto during the Clone Wars and ending with the assault on Hoth as depicted in Star Wars Episode V: The Empire Strikes Back. There are a total of 18 missions, three of which are optional space missions.
The PSP version of Battlefront II does not feature the Rise of the Empire campaign. Instead it features three single-player Challenge modes; Imperial Enforcer, Rogue Assassin, and Rebel Raider. In Imperial Enforcer the player is sent to eliminate indigenous species on several given planets, such as Gungans on Naboo and Ewoks on Endor. Rogue Assassin requires the player to eliminate all Imperial officers on a given sets of planets. Bonus points are awarded for any other kills. Finally, Rebel Raider tasks the player with locating specific objects in a map and returning them to a designated drop point, similar to Capture the flag.
Like its predecessor, Battlefront II includes Galactic Conquest. In this mode, the player commands a fleet throughout the galaxy conquering and protecting planets, much like a game of Risk. When two opposing forces reach the same planet, the game switches to the traditional perspective, and the player must eliminate the enemy faction to gain control of that planet. Players gain credits for performing well which can be used to buy new character classes, a new fleet, or bonuses that provide additional support when attacking or defending a planet. While the PlayStation 2 and Xbox versions allow cooperative and competitive battles in Galactic Conquest mode through splitscreen gameplay, the PC version does not.
In Battlefront II players can battle in space and engage in ship-to-ship combat. Players sabotage enemy capital ships externally by firing at vital systems, or on foot by landing in the enemy hangar.
In Instant Action players can choose from any of the game's levels, as well as any available eras and modes. Four other modes are included in addition to the traditional Conquest mode that was found in Star Wars: Battlefront; Hunt mode, Capture the Flag (CTF), which is available in 1-flag and 2-flag variants, Hero Assault and Space Assault.
In Conquest mode, players are required to capture command posts. Players capture command posts by standing near one until its holograph turns blue. Players can capture both enemy command posts and neutral command posts. For enemy command posts, enemy units can continue to spawn in the area until they are de-energized. When all command posts belong to a given team, a twenty second timer begins in which the opposing team must de-energize a command post or they lose. Alternatively, if one team eliminates all opposing reinforcements, they win.
In Hunt mode, players take on the role of the species indigenous to the chosen planet, or of a faction opposed to that species. The object when playing as the indigenous species is to repel the opposition, or to eliminate the indigenous species if playing as the opposing faction. In 1-flag CTF, both teams attempt to take a common flag and capture it at the enemy's base. This mode is the only one available on ground maps as well as in space. 2-flag CTF tasks players with stealing the enemy's flag and returning it to their base for points.
In Hero Assault players control iconic Star Wars characters which are divided into two teams, heroes and villains. The sole objective is to be the first team to reach the number of required kills. Space Assault allows players to control a starfighter in order to destroy critical systems on the enemy's capital ship or destroy other enemy starfighters for points. The systems of an enemy capital ship can be destroyed in a starfighter by firing at key areas of the ship. Alternatively, players can land in the enemy hangar and sabotage critical systems internally. The game ends when one team has reached the required number of total points.
The game is told as an autobiography, with an unknown clone recounting the battles of the 501st Legion. The 501st begins as part of the Old Republic, and fights against the Separatists across the galaxy. The campaign begins with Chancellor Palpatine commanding the 501st to destroy an advanced energy generator on Mygeeto, but secretly he wants them to collect the remaining samples to help power his future space station, the Death Star. Meanwhile, Palpatine himself is captured by the Separatists, and the 501st clears the way for Obi-Wan and Anakin Skywalker to rescue him. On Felucia, the 501st exterminates a small infestation of Acklay inhabiting the planet and eventually eliminates the CIS presence in the area. Afterward on the planet Kashyyyk, the clones engage a huge CIS fleet in space, while planetside they defend a vital Wookiee base from a numerically superior enemy army. With the support of Wookiees and Jedi Master Yoda, the invasion is repelled. In the final mission of the Clone Wars era, the clones strike a CIS stronghold on the planet Utapau, killing separatist leader General Grievous, with the help of Obi-Wan Kenobi.
Immediately after the victory on Utapau, Palpatine orders the execution of Order 66. The 501st participate in the destruction of the Jedi Order on Coruscant, and massacre the remaining Jedi knights guarding their temple with the help of the new Sith Lord, Darth Vader. With the newly established Empire in power, the 501st becomes known as Vader's Fist, Darth Vader's personal army. Under their new leader the 501st is assigned to various missions to strengthen the Empire's power. They are first tasked to force a regime change on Naboo by assassinating the Queen and surviving Jedi knights seeking protection. Shortly after, the 501st are called to destroy a droid manufacturing facility on Mustafar that was reactivated by a Geonosian known as Gizor Delso. Learning that the Kaminoans have created a new batch of rogue clones, the 501st is ordered to destroy the cloning facility on Kamino. The 501st battles the altered clone army, gaining victory with the help of the bounty hunter Boba Fett.
With the Galactic Empire firmly established in the ashes of the Old Republic, the 501st receive orders to be stationed on the Death Star. During their watch a prison break is initiated, with crucial plans being stolen by the rebels. Tasked to recover the stolen plans, they search a rebel base on Polis Massa, but the plans are nowhere to be found. The search eventually leads the 501st to the Tantive IV starship. Despite capturing the Rebel sympathizer Princess Leia, the plans are still transmitted, and the Death Star is destroyed with many soldiers still stationed inside. The Empire retaliates by sending the 501st legion to Yavin IV, where a rebel base is located. In the final mission, the 501st is pressed into crushing the weakened Rebel Alliance. The clones defeat the rebels and capture Echo Base, leading to the victory on Hoth. The clone narrator proudly proclaims the Rebellion finished, though the game follows the plot of The Empire Strikes Back, with the Millennium Falcon escaping Hoth.