Dain is Thorin’s cousin, and the battle begins because Thorin assumes that Bard is going to steal his treasure.  Smaug the dragon is hoarding treasure that once belonged to Thorin the drawf’s father.  When Bard kills the dragon, the men and elves head for the dragon’s lair.  Thorin assumes that the men and elves are going for his treasure.  Dain's army attacks when Bard's army appears.  This is because the Dwarves are there and ready for war, and they see Bard's arrival as threatening.  Bilbo gives the Arkenstone to Bard, hoping that he can use it to negotiate peace with Thorin.  Thorin assumes Bilbo betrayed him, and the stone only further angers him.

Once Gandalf arrives, he tells them that there are goblins and wargs coming to attack both sides.  This is the beginning of the Battle of Five Armies, because the elves also join the fray.  Everyone is fighting together then, against the goblins 

Dain is Thorin’s cousin, and the battle begins because Thorin assumes that Bard is going to steal his treasure.  Smaug the dragon is hoarding treasure that once belonged to Thorin the drawf’s father.  When Bard kills the dragon, the men and elves head for the dragon’s lair.  Thorin assumes that the men and elves are going for his treasure.  Dain's army attacks when Bard's army appears.  This is because the Dwarves are there and ready for war, and they see Bard's arrival as threatening.  Bilbo gives the Arkenstone to Bard, hoping that he can use it to negotiate peace with Thorin.  Thorin assumes Bilbo betrayed him, and the stone only further angers him.

Once Gandalf arrives, he tells them that there are goblins and wargs coming to attack both sides.  This is the beginning of the Battle of Five Armies, because the elves also join the fray.  Everyone is fighting together then, against the goblins