In this fanfic, Harry could declare war on Umbridge by leading a series of protests. He might start with a simple act like wearing badges that say 'Down with Umbridge'. As things escalate, he could organize a large - scale walk - out during one of her particularly unfair lessons. Harry would likely use his knowledge of the school's secrets and passages to outwit Umbridge and her minions, while also trying to protect the students from her retaliation.
The plot might involve Harry finding allies among the teachers who also dislike Umbridge. Together, they could plan ways to undermine her authority. Harry could use his skills in Defense Against the Dark Arts, which Umbridge has been suppressing, to train the students secretly. This would be like his own form of declaring war. He could also expose Umbridge's incompetence and cruelty to the wizarding community outside of Hogwarts, causing a stir and putting pressure on her to leave.
Well, it could be that Harry gets tired of Umbridge's tyrannical rule at Hogwarts. He might start by secretly organizing the students to resist her unfair rules. Maybe he uses his status as the Boy Who Lived to inspire others. For example, he could encourage students to defy her ban on certain magical activities. Then, he might start a campaign of small acts of rebellion, like writing anonymous letters about the truth of what's happening in the school.
Some of the older students like the Gryffindor prefects could support Harry. They understand the importance of a just Hogwarts environment. Neville Longbottom could also be a strong supporter. He has grown a lot in courage throughout the series, and this would be his chance to show his loyalty to Harry and fight against Umbridge's unfair treatment of the students.
Perhaps in this fanfiction, Harry Potter declares war because he discovers a new and even more dangerous threat that the wizarding world has been ignoring. He realizes that the only way to deal with it is to take a stand and fight. So, he uses his influence and skills to form an army and begins a full - scale war against this new enemy, which might be a powerful dark wizard from a far - off land or an ancient evil that has been awakened.
In this fanfiction, it could be that Undyne declares war against an unjust regime. She might gather an army of like - minded individuals and engage in battles that not only test her physical strength but also her strategic thinking. The story could detail her training of troops, her alliances, and the challenges she faces on the battlefield.
One possibility is that Umbridge, despite her cruel nature, sees some potential in Harry for her own gain. Maybe she tries to mold him into a perfect little wizard according to her strict and often wrong ideas. She could isolate Harry from his friends and force him to follow her rules blindly, which would create a lot of internal conflict for Harry as he tries to resist her brainwashing while living under her roof.
It could involve Harry Potter taking a stand against a dark wizarding family that has wronged his loved ones. Maybe they've targeted his friends or family, and he decides that a blood feud is the only way to deal with their ongoing threats and attacks.
It could be that the fanfiction used Harry's likeness without permission in a way that was harmful to his image. Maybe it had Harry involved in illegal or immoral activities that he would never do in the original series, so he decides to take legal action against the author of the fanfic.
One possible consequence could be that she gets fired from her position. If Harry manages to expose her actions, the wizarding school or the Ministry of Magic might not tolerate such blatant abuse of power and take disciplinary action against her.
Hermione and Ron would surely be his top allies. Hermione with her vast knowledge of magic and Ron with his loyalty and combat skills. They've been with Harry through thick and thin. Also, the members of the Order of the Phoenix like Sirius Black (if he's still alive in the fanfiction), Lupin, and Moody could be great allies. They have experience and are very powerful wizards.
One major consequence could be that Harry reveals his connection to Sirius Black. If Umbridge gets to know about this through the Veritaserum, she could use it as a weapon against Harry. She might try to capture Sirius or use his existence as a way to blackmail Harry into following her every order. Also, Harry could end up revealing the details of how he has been learning magic outside of the school curriculum, which would be a huge violation in Umbridge's eyes.
One possible plot could be that Harry and Hermione travel the wizarding world together, documenting all the post - war changes. They might visit Hogwarts, which is being rebuilt, and help in the restoration process, reminiscing about their old school days as they do so.