So you are someone that wants to fight in battle for the greater good, willing to face risks and danger. You dedicate your whole life to battle and fighting. So, you want to fight but not for a country, or a ruler. You don't want to take orders and follow the ideas of a country but want to fight for the overall greater good of humanity.
What would you sign up for, who would you go to, where would you go. 
Well the US military is where you go to learn how to be an effective killer. Street gangs actually tell members to enlist and then teach the gang when they get out. Army or Marines. You can do that stuff in the Air Force and Navy, but it is much more hit or miss. If you're in shape, you can get into a community that sees combat and isn't just a grunt.
After that, you can go freelance. In the military, you're going to be disappointed by how much button and pencil pushers run the show.
There is no military that fights for the greater good of humanity. You don't go to war for that shit. GIJoe isn't real. You go to battle for your brothers in arms. You will never bond with someone until you've gone to battle with them. 
Most young men get hustled in to wars. This is nothing new. 90% of casualties of 21st century war have been civilian. Might be accurate and might not be. Just a stat I cam across on the interwebs.
How many soldiers have it in them that they could be trained to crush a child's testicles to extract information from the mother regarding the were abouts of the father? Read that shit went on in Iraq.
I think I'd rather take a bullet to my dome than crush a child's nuts but I'm sitting on my ass at a computer in a 1st world civilian environment and have not been put through the training that makes men killing machines.