Right?
1 country in the world won't make you safe from wars. There's a reason why military was and is separated from police. Just think what's going to happen when your soldiers start to kill citizens.
1 country in the world tend to degrade to dystopian regime, and it was researched by multiple books authors, and actually a modern day USA is a good example of a regime on the path to dystopia.
This is the logic behind the old-style Jihad. Also the Chinese emperors used the same argument when conquering smaller kingdoms.
Unfortunately, what happens in practice is that even in one Islamic country Sunni vs Shia terrorists attacks happen. And in the case of China, the price you pay for superficial peace are Uighur concentration camps, Tibetan self-immolations and so on.