The United Nations dedicates itself to maintaining peace and stability throughout the world, and occasionally sends in peacekeeping troops to protect civilians. The U.N. doesn't maintain its own standing army. Instead, U.N. peacekeeping troops are members of their home country's military who agree to serve in the U.N. military for a year.
Join the Military
To be eligible to become a U.N. soldier, you must first become a soldier in your home country. Eligibility requirements vary from country to country. In the U.S., you must be be at least 17 and pass the Armed Services Vocational Aptitude Test. Each branch of the military sets additional requirements. The U.N. uses peacekeepers from all military branches, so there's no single military branch that will guarantee you a spot in the U.N.
If you opt to join the U.S. military in the hopes of joining the U.N., there are no guarantees. Participation in a U.N. mission depends on whether your platoon is recruited to work with them. This is primarily dependent on whether the U.N. has a current mission that your platoon is qualified to perform. Rarely, you may be able to ask your commanding officer to transfer you to a different unit, but transfers are rare and there is no form or guaranteed process for initiating a transfer. Instead, being recruited to serve in the U.N. is largely a product of chance, so becoming a U.N. police officer may be a better bet.
Become a Police Officer
Police officers are an important part of the U.N. military. Of about 98,000 uniformed personnel deployed worldwide as of July 2014, about 12,000 are police officers. To work in the U.N.'s military police units, you must become a police officer in the United States, then accumulate five years of police experience. To become a police officer -- not just to work for the U.N. -- you will need to attend your local police academy and then become certified by your state's Public Safety Officer Standards and Training Council.
Applying for Work
To serve as a U.N. police officer, you have to apply directly for such a role through one of their private contractors. You'll need to pass a physical fitness test that includes measures of agility such as the dummy drag -- a physical fitness measure during which you must drag a dummy across a field, through an obstacle course or over a predetermined distance. If you're accepted, you then go to the U.N. for additional training, and then you are deployed to one of the U.N.'s peacekeeping missions.