Well, he might be initially confused. Naruto comes from a world with ninjas, not a modern army base. He could think it's some new kind of enemy stronghold. Maybe he'd try to infiltrate it in his typical ninja way, using stealth and jutsus.
In a fanfic where Naruto finds an army base, he might initially be on high alert. He could use his ninja skills to scout the area stealthily. Maybe he would try to figure out if the army base is a threat to his village. If the people in the base seem hostile, he would likely be ready to defend himself using his jutsus like the Rasengan.
Some of the soldiers in the base might be scared. Naruto's appearance, with his orange jumpsuit and strange ninja tools, would be really out of the ordinary for them. They might think he's an alien or some sort of super - powered threat.
Another possibility is that he discovers a hidden room with a secret about the history of the ninja world. This could be related to a long - lost clan or an old war that has been forgotten. Naruto would then have to decide what to do with this new information, whether to share it with his friends or keep it a secret for the time being.
Sasuke would be more aloof about it. He might not show much interest on the surface, but secretly he'd be observing Naruto's every move. If there was something in the base that could help him in his quest for power, he'd definitely be interested.
One main theme is leadership. Naruto has to step up and lead the newly - found army. He has to make decisions and inspire the members. Another theme is unity. The army, which may consist of different types of people or beings, has to come together and work as one. For example, if there are members from different villages in the army, they need to put aside their old rivalries.
Well, I'm not sure of the exact name, but there's a 'naruto finds an army' fanfic where Naruto finds an army of ancient warriors. These warriors were sealed away long ago and Naruto accidentally releases them. At first, they are hostile, but Naruto manages to convince them to help him fight against a common enemy. The fanfic is really interesting as it combines elements of Naruto's world with ancient legends.
Naruto would likely approach the army base with caution. He's a brave character, but not reckless. He'd use his sensei's teachings to figure out how to handle this unexpected discovery. He might send out a clone to scout ahead while he stays hidden and observes.
One possible plot could be that Naruto is on a mission in a mysterious land and stumbles upon the Golden Army by accident. Maybe he initially mistakes them for some kind of powerful jutsu - created constructs. As he tries to figure out what they are, he discovers their unique powers and abilities. He then has to decide whether to use them for the good of his village or leave them be as they might be a dangerous unknown force.