Well, one possibility could be that Naruto is somehow influenced by a dark force. Maybe he discovers some dark secrets about the shinobi world that make him believe war is the only way to bring about true change. He might start by attacking the Five Great Nations, thinking he can overthrow the existing power structures and create a new, more just world.
Perhaps Naruto starts a war because he's been misled. He could be tricked into believing that another village is planning a massive attack on Konoha and all of its allies. In his haste to protect his home and friends, he launches a preemptive strike, which then spirals into a full - scale war as other villages retaliate.
In a fanfiction scenario where Naruto starts a war, it could be due to a personal vendetta against a certain group. For example, if a group of rogue ninjas were responsible for the death of someone he loved dearly, like Jiraiya. Naruto's anger and grief could drive him to wage war against all those associated with that group, regardless of the consequences for the larger shinobi world.
Chapter 590, "Are You Angry?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, Night had completely fallen, and a chill hung in the air over Dragon.... By the end, The moonlight enveloped her, making her already slender figure appear....
Chapter 835, "Is this your way of handling things? So pathetic.", is the clearest early answer because it turns Chinese's situation into a concrete plot event. A useful checkpoint is the way the scene starts with In a private teahouse in the Capital, a few bejeweled wealthy ladies...
Chapter 555, "Peaceful Coexistence", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. It opens with If you don't want to be discovered, then simply don't do it," Bai Lu... and closes on We cannot underestimate their strength, and today we chose defense..
Chapter 564, "Why Only 80 Points?!", is the clearest early answer because it turns Ai Xiaochun's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Miss Jiang, what do you need this time?" He Pingzhang's tone was not...
Chapter 559, "This is the Worldview of the Main Female Lead", is the clearest early answer because it turns Ai Xiaochun's situation into a concrete plot event. It opens with You!" Ai Xiaochun gasped, unable to speak, her teeth clenched as she...
Chapter 626, "Emergency PR!", is the clearest early answer because it turns Wilderness Survival's situation into a concrete plot event.
Chapter 735, "How is it Him?!", is the clearest early answer because it turns Chang's situation into a concrete plot event.
Chapter 390, "Is Jiang Li Commended by the Police Again?!", is the clearest early answer because it turns Is Jiang Li's situation into a concrete plot event. It opens with The bell rang for school to let out, and Tao Ying hurriedly grabbed...
Chapter 582, "Fleeing Hastily", is the clearest early answer because it turns Luo Xuan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Luo Xuan pushed her away, disappointment filling his eyes.
Chapter 586, "She Just Likes Men Who Show Weakness Initiative", is the clearest early answer because it turns Gu Yu's situation into a concrete plot event. The chapter turns that into something concrete by moving from Jiang Yan quickly swam over to the person.