Persians With Pistols
The military history of Iran dates back millennia. Even before settling in what is now Iran, Indo-Iranian tribes had invented the chariot by around 2000 BC. During their expansion into Iran and Central Asia during the early first millennium BC, Iranians developed the concept of cavalry, being the first people in the world to ride horses in battle on a large scale. Ancient Iranians were also noteworthy for the participation of women in battle; up to 20% of some tribes' warrior graves have female remains.
The Achaemenid Empire, which at its peak ruled over nearly half of the world's population, was known for the huge number of troops it fielded. Though ancient sources were prone to exaggeration, the Achaemenids could raise armies on an unprecedented scale. The most well-known Achaemenid unit was the Immortals, an elite 10,000-man corps that was so named because each casualty was immediately replaced, keeping the unit size constant.
The Achaemenid Empire was conquered by Alexander the Great, but a few decades later the Parthians, an Iranian tribe originating in what is now Turkmenistan, revolted against Greek rule and eventually conquered the Iranian plateau and Mesopotamia. The Parthian army was based on a one-two punch of horse archers and heavy cavalry.
The Sassanid dynasty from Persia replaced the Parthians in AD 224. The Sassanids were known for their elite heavy cavalry, the Savaran. They were The Rival to The Roman Empire, and after a particularly exhausting war with Rome they were conquered by the Arabs.
For centuries later, Iran was ruled by Arabs and then by a succession of lesser Iranian and Turkish kingdoms until the rise of the next Iranian empire under the Safavids. The Safavid army was originally centered on the Qizilbash Turkmen which brought the dynasty to power. Later on, the Shah added musketeers and artillery in order to keep up with the Ottomans. The collapse of the Safavids led to disorder which eventually caused Iran's territory to shrink to its current borders.
The modern Iranian army's history starts with Reza Shah's military reforms. He sent his officers to train in western academies and also brought in foreign advisors to train his soldiers. Reza Shah also established an air force and a new navy.
After the Iranian revolution, the new authorities established the Revolutionary Guards and the Basij, both of which primarily deal with internal security. The Revolutionary Guards are more of an army within an army (they have their own navy and air force) while the Basij are a paramilitary militia under nominal control of the Revolutionary Guards. Iran also developed its domestic arms industry after the revolution, as it found itself diplomatically isolated.
The last major war Iran has fought is the Iran-Iraq war. It was a very bloody war which eventually ended with no gains for either side, but it had a profound effect on Iranians, who lost up to 1,000,000 people in the war. Both sides used Child Soldiers, though the use of children in the military was illegal under Iranian law. Iranian religious fervor and nationalism were very intense during the war, in some cases resulting in volunteers absurdly charging into minefields in order to clear paths for other soldiers.
Nowdays, many people accuse Iran's government of attempting to build nuclear weapons, though Iran, of course, vehemently denies this.
In western works, Iran's military usually serves as a generic Middle Eastern antagonist, being almost a textbook example of an Acceptable Target.
Also see Iran.