The Rise of Darkrai
Dialga was battling against its enemy, Palkia. During the battle, it used Roar of Time on Palkia which wounded it's left arm and caused it to flee into Alamos Town. Due to Palkia's arrival, the Pokémon who were having nightmares under the influence of Darkrai's Dark Void woke up. Simultaneously, Palkia's presence causes the dimension Dialga and Palkia were fighting in to disintergrate the normal dimension little by little. Dialga, who was furious at Palkia for its cowardice, engaged it to continue the battle which did enormous damage to the entire town. During the battle, Darkrai tried to hold them off as long as he could, but ended up getting himself killed in the process of saving the town. Before Dialga and Palkia can deliver the final blow against each other, Ash and Dawn successfully managed to magnify the volume of the song, Oración, soothing both Dialga and Palkia in mid-flight and healing Palkia's wound. Just as Dialga returned to its dimension, Ash and Dawn asked Palkia to return Alamos Town back to its original dimension. Once everything was restored, Palkia returned to its dimension.
Giratina and the Sky Warrior
Dialga played a small role. In the beginning of the film, it appeared near a lake and gets a drink from it. But little did Dialga know, Giratina's Reverse World was infected with dark and poisonous clouds which were caused by its previous battle against Palkia from the preivous movie. The influence from the battle angered Giratina and made it believe that both Dialga and Palkia are the cause of the Reverse World's devastation. Giratina lures Dialga into the Reverse World and engages it to battle. Thankfully, Shaymin's Seed Flare gave Dialga a chance to escape the Reverse World and return to its dimension.
Arceus and the Jewel of Life
Dialga was summoned by Sheena to rescue Pikachu and Piplup from being sucked into a deadly spatial distortion and closes it up with Roar of Time. Moments later, Giratina arrives and tries to battle Dialga. Sheena was unable to reach into Giratina's heart due to its anger. Ash successfully calmed it down by pleading it to stop after recognizing him as the same person that saved it from Zero from the previous film. After Sheena reached into Giratina's heart and told it to return to its dimension, the spatial distortion opens again and this time Dialga gets itself caught in the water spouts. Just as all hope seemed lost for Dialga, Palkia arrived and rescued Dialga by using Spacial Rend on the spatial distortion to close it. Sheena then connected with the hearts of both Dialga and Palkia and puts an end to their conflict with each other once and for all. After that, Dialga and Palkia returned to their respective dimensions. Sheena and Kevin explained to Ash and his friends that the reason why Dialga and Palkia clashed in Alamos Town was because both of their dimensions, which were not meant to ever intersect, intersected due to Arceus's preparation to awaken from its long, eternal slumber. As a result, both Dialga and Palkia believed that they both invaded each other's territory and the battle also affected the Reverse World which made it bring Giratina into the conflict as well. When Arceus arrived and prepared to destroy all of the humans due to Damos' betrayal, Dialga, Palkia, and Giratina arrived and protected Ash and his friends from Arceus's attacks. As Palkia and Giratina holds Arceus off, Dialga used its temporal abilities to send Sheena, Ash, Dawn, Brock, Pikachu, and Piplup back in time to give them a chance to change the past and save the future. Dialga was then badly injured by Arceus alongside Palkia and Giratina, and becomes exhausted due to time travel. Eventually, when Ash and the others had successfully changed the timeline and saved the future, Arceus healed Dialga and the others and Dialga returned to its own dimension to live peacefully once again.
Dialga seems to be a very fierce battler, possibly valuing an complete battle over its initial objectives. As seen in The Rise of Darkrai, it's first reaction on meeting Palkia was to get involved in a humongous territorial battle. But even after Palkia was hurt and Dialga had already expelled it, which was it's initial intention, it still give chase to Palkia to finish the battle, seemingly very enraged once it discovered Palkia had fled.
Dialga also seems that it may be unwilling to fight determined enemies, just as shown in Giratina and the Sky Warrior, when it was ambushed by Giratina and still tried to flee several times instead of fighting it directly. Even when Dialga perceived it would have to defend itself to escape from Giratina, it still decided to lock it in a time loop to prevent pursuit instead of attacking it. After that, it fled once and for all.
Finally, Dialga seemed to be willing to help sometimes, as shown when it saved Dawn's Piplup and Ash's Pikachu from the space-time vortex. The same when it used Protect to save Ash and the others from Arceus's Judgement. It also didn't hesitate in sending Ash and Co back int time to prevent Damos alleged betrayal.
It could be understood that Dialga can be willing to forgive, establishing peace and possibly some kind of friendship, given Palkia didn't attacked it and saved it willingly without even being asked to do so.
Despite having a tendency to battle, it was shown that it don't have a clearly sense of strategy, not caring not even a little about it's moves's negative effects. It was seen to have chosen and maintained it's desired move set based exclusively on sheer power, since it knows two moves that requires a recharge turn and one that reduces harshly it's Sp.Attk every time it's used. Plus, it didn't bothered learning a Steel-type move, even being stab and good against Fairy and Ice types.
|Roar of Time||The Rise of Darkrai|
|Draco Meteor||The Rise of Darkrai|
|Hyper Beam||The Rise of Darkrai|
|Protect||The Rise of Darkrai|
|+ indicates this Pokémon used this move recently.*|
- indicates this Pokémon normally can't use this move.