Even though nowadays many countries are struggling to protect peace, there are several official "war" conflict zones on Earth. In many cases, major countries can interfere with their own troops. However, in order to prevent the risk of losing soldiers from a national army, or in case a country doesn’t have enough of a military force, a government can hire mercenaries.
Mercenaries are soldiers who are fighting in exchange for a gain or material compensation. If we look back in the history, such military force was used for thousands of years in Ancient Greece, Egypt, and the Roman Empire.You may take a look at Insider Monkey for the largest private armies in the world.