US marine: 'I am not worried about going to war'
14 January 2014 Last updated at 18:52 GMT
Across the globe there are men and women fighting in many conflicts.
In the US, serving in the military is a way for young people to get a subsidized university education.
The BBC's Regan Morris went to San Diego to meet with an 18-year-old marine to ask him why he joined the military.