There are a number of laws regarding wars, and when it is and is not permissible to engage in hostilies against another country.

Wars of aggression are immoral by default (if you subscribe to a religious moral framework, like the laws of Moses or the teachings of Jesus or the word of God in the Qur'an, at least, but I'm sure even Western secular movements oppose murder), but international and American law are a bit more flexible, I guess.