TSC's answer is a good one, but not exactly accurate. Historically, serving in the military has been something people did either by choice for any number of reasons (money, training, lack of job, sometimes 'patriotism', etc.) or because they were compelled to do so (conscription/draft, forced servitude, slavery, etc.). We like to believe that people 'serve their country' voluntarily, but it really depends on the cause. For example, in WWII, about 16 million Americans served (1/10th of the population). The cause was just, regardless, most service members were drafted. Same in WWI and even in the Civil War.

And things aren't always what romantic history makes them out to be. For example, Were you asked, "In 1776 would you have been a Rebel or a Tory (British loyalist)," odds are you would say "Rebel." But fact is that in 1776 the vast majority of "Americans" were Tories. People such as Sam Adams and Paul Revere were outright traitors as far as the British Crown was concerned, and they made up stories and told outright lies to inflame the public (the alleged "Boston Massacre" - which was no such thing - is a classic example).

Today the Armed Forces of the United States are "all-volunteer" - that's fine, but an all-volunteer military is definitely different from a "citizen-soldier" armed forces. When soldiers are "hired" to fight, vs. the WWII motivation for example, it often boils down to letting someone who wants to fight do it.

Perhaps TSC forgets that when he was 18, there was great dissension in the land regarding our involvement in Vietnam. As we later learned, the "Tonkin Gulf Incident", which was President Johnson's justification for escalating the war, was a lie. Most of the soldiers who fought in Vietnam were drafted. They did a great job.

For what it is worth, I spent nearly 31 years in the Army.