Naruto would face the psychological challenge of fighting alone. He is used to having his friends by his side. Without their support, he might have moments of self - doubt. The army might also use psychological warfare against him. They could taunt him or try to make him angry to disrupt his focus. In terms of actual combat, the coordination within the army could be a big obstacle. If they are well - organized and work together effectively, they could set up complex traps and ambushes for Naruto. And if the army has access to some sort of large - scale destruction jutsu, Naruto would have to find a way to either avoid it or counter it.
Well, Naruto has a lot of powerful jutsus at his disposal. His Rasengan alone can cause a great deal of damage. He could also use his shadow clones to create distractions and attack from multiple angles. And let's not forget his Nine - Tails chakra. With that, he could boost his strength and speed to take on a large number of enemies at once.
One challenge is trust. Not everyone will immediately believe in Naruto's intentions. Some may think he is trying to gain power for himself. Another challenge could be resources. Feeding, housing, and training an army requires a lot of resources. He needs to find ways to get enough funds and supplies.
One challenge is controlling the amount of natural energy. It's not easy to find the perfect balance. Too much and he could turn into a statue or something similar. Another is that he doesn't have a proper teacher to correct him immediately. He has to rely on his own instincts and the limited knowledge he has about senjutsu.
One challenge could be overexertion. He might push his body too far and risk injury. In fanfiction, Naruto's intense training might also attract unwanted attention from enemies who see his growth as a threat. Another challenge could be self - doubt. Despite his determination, there might be moments when he questions if his efforts are enough to reach his goals.
His friends from the Hidden Leaf Village might help. For example, Sasuke could be a powerful ally. He has his Sharingan and his own set of powerful jutsu.
In fanfiction, if Naruto goes to Kiri, he would face challenges related to the village's internal politics. There could be different groups within Kiri with their own agendas, and Naruto's presence might disrupt the balance. He might also struggle with the language barrier or cultural differences. For example, some of Kiri's traditions might be completely foreign to him, and he could accidentally offend someone. And of course, there are always the powerful and dangerous missing - nin from Kiri that he could run into.
He would face a huge ideological challenge. Madara has very extreme views about power and the ninja world order, which are completely different from Naruto's beliefs in friendship and cooperation. Naruto would constantly have to struggle not to be swayed by Madara's ideas.
He would face political challenges. Naruto is used to the relatively straightforward hierarchy in the ninja world. As a daimyo, he has to deal with complex political intrigue among the nobles and different factions. For example, there could be groups vying for power and trying to manipulate him.
In this fanfiction scenario, Naruto would face the challenge of changing the perception of the Uchiha within the village. Since he is now part of the clan, he would want to show that the Uchiha are not to be feared. However, there would be those in the village who still hold grudges or suspicions towards the Uchiha. Also, Naruto would need to manage his relationship with Sasuke in a new way. They are now family, but Sasuke has his own path and issues. Naruto would have to find a way to support Sasuke while also upholding the values of the Uchiha clan. Additionally, he would have to face the internal politics of the Uchiha clan itself, which could be complex and full of power struggles.
One challenge could be facing powerful new enemies. In fanfiction, authors often create unique and formidable foes for Naruto. These enemies might have strange powers or abilities that Naruto has never encountered before, forcing him to quickly adapt his fighting style. Another challenge could be the moral dilemmas. He might have to make tough decisions in the chaos of war, like whether to sacrifice a few to save many.