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.
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.
In the context of the original Harry Potter story, it seems quite abnormal as Umbridge is a very unlikable character to Harry. But in the world of fanfic, anything can happen. Fanfic allows for different interpretations and character developments, so while it's not normal in the traditional sense, it's possible within the creative space of fan - created stories.
Another way Harry could respond is with a sense of shock and disbelief at first. Given that Umbridge is supposed to be in a position of authority, Harry might be momentarily stunned that she would go to such lengths. However, his Gryffindor courage would soon kick in, and he would likely start to resist, perhaps by using his magic in creative ways to protect himself or to disarm Umbridge, all while trying to uphold the values of the wizarding world that oppose such cruel and unjust treatment.
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.
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.
Perhaps in the fanfic, Harry teams up with the other students to stage a large - scale protest against Umbridge. Their united front and the chaos it causes might lead to the school board or the Ministry of Magic realizing that she is not fit to be in charge at Hogwarts. Or Harry could use his connections, like his friendship with Sirius or the Order of the Phoenix members. They could dig up some dirt on Umbridge from her past that makes it impossible for her to continue teaching at Hogwarts.
In 'umbridge abuses harry fanfic', often Umbridge is portrayed as a tyrant. She might verbally abuse Harry, constantly doubting him and his claims about the Dark Lord. For example, she could publicly humiliate him in front of other students, and try to isolate him. This all builds on the canon idea of her as a very unpleasant and unjust character towards Harry in the original story.