There are many times when countries need to go to war. There are also times when war was not necessary but carried out none the less. The United States has made decisions just like other countries, where they think they are helping their people, when in reality they are hurting themselves, and others.
One situation where war is in dire need would be if the country was threatened by another country. For example: being threatened to be bombed, over run, anything that would over-throw the current government. This is a time where it's either loose the country, or fight the battles and win what's rightfully the ownership of the country. They can't just let people walk in and step all over them. Nations have to stand up against these things. .
In addition to why people should commence in war, there are reasons why countries should not go to war. For example: Civil war is not needed. There is no reason that people should go to war within themselves because what they believe is right. Well that's a bad example. Some civil wars are just because certain people don't agree with laws. There's no reason to rebel on your country because there are law which you can not abide by. Also, there shouldn't be wars for oil, or any other natural resource. People should be able to work out those confrontations behind closed doors, not out on a battlefield taking lives. That's a few reasons why there shouldn't be war.
Finally, the last example on why a country should commence in war would be the situation the British and the Allied forces faced back in World War II. They had a group of people, well a nation that was set on genocide or in other words, making a dominant race. Without they help of nations around Europe, Hitler would have blown right through Europe. If not for the actions to protect their nations, Hitler would have succeeded, and almost did. That's a reason to go to war, to protect what is rightfully yours.