very interesting question.
I think its basically because the nation has more power than individuals. Power and Wealth allows people to do whatever they want. But still for modern nations to go to war they need an excuse and nations would always find a way to justified their country by finding excuses.
For example In During WWI time, the 5 Great nations of Europe were trying to compete for land (Empire), raw material, and more technology. Its basically the country's greed and jealous that cause wars. During WWI they send out a lot of nationalism, which make people want to join the army and want to fight for their country. This nationalism make people feel like they are contributing to their country,family and people, and it also made the arm race between the countries possible. Large army cause more tension between the countries and made war easier to start. Basically a Serbian killed the prince prince of Austira-Hungary, then a month later Austira declare war on Serbia, But Sebria had alliance with Russia;
Russia war with Austira
Austira has alliance with Germany, Germany war with Russia
Russia has alliance with France, France war with Germany
France had alliance with British, British war with Germany
You see the pattern? they all have an excuse for war.... Well it was a lot of Kasier Wilhelm's fault, because he went against all the peaceful reforms that Bismark did and make all the countries felt that Germany was a big threat.
Anyways Modern example would be United States....
I'm not anti-American but what Bush administration did was more then revenge for the 9/11, they basically invade the whole country and placed all our solider to take over their oil factories.
So its okay for Nations to go to war because they are more powerful the individuals, they have "good excuse", and simply going to war is fighting for their country, protecting their land, they are lot of things... but the main idea is because they are powerful but remember this, its usually winner rewrite the history.