Presumably, the first tussles between humans were mostly unorganized, spontaneous skirmishes. But at some point, people thought to organize a group of talented fighters from their midst in order to protect themselves and/or for aggression. These were the first armies, filled with the world's first soldiers.
Military historians trace these early fighters to Sumer, which first existed around 4000 B.C.E. One of the oldest urban civilizations on Earth, the city-state was located in what is today Iraq. The first recorded war was from 2525 B.C.E. and was about two cities in Sumer fighting over possession of the region of Guendena (sound familiar?) The Sumerians, who fought constantly over 2,000 years, also invented the helmet, the chariot and the sickle sword (a sword with a crescent-shaped blade) among other military gear [sources: The Air University, Swirk]].
From there, soldiers spread to every corner of the world. Some of the more notable forces over time were the Mongol Army, 1 million strong, which conquered most of Eurasia starting in 1206; the Roman Army, which similarly corralled the Western world; and the Ottoman Army, which subjugated people in the Middle East, Balkans and North Africa for 500 years, until the 19th century [source: Keck and Pillalamarri]. With nearly every nation on Earth in possession of an army, "soldier" is almost assuredly a profession that is here to stay.