Here's why:
* The Battle of Hogwarts: The climax of the book is a massive battle at Hogwarts School of Witchcraft and Wizardry, where Harry, his friends, and the Order of the Phoenix fight against Voldemort's army.
* The Second Wizarding War: The entire book takes place during the Second Wizarding War, which is the culmination of the conflict between Voldemort and the wizarding world.
* The Death Eaters' takeover: Voldemort and his Death Eaters have seized control of the Ministry of Magic and are terrorizing the wizarding world, forcing many wizards and witches to flee into hiding.
So, while there are smaller battles throughout the book, the 7th book is ultimately a narrative of warfare, sacrifice, and the ultimate struggle for good versus evil.