The previously mentioned early Indo-Iranian kingdoms and statehoods were to a large degree the ancestors of the north-eastern Iranian tribes and the Medians, who would found the first Iranian Empire in 625 BC. It was the Median Empire that left the first written proof of horse breeding around the 7th century BC, being the first to propagate a specific horse breed, known as the Nisean, which originated in the Zagros Mountains for use as heavy cavalry. The Nisean would become renowned in the Ancient World and particularly in Ancient Persia as the mount of nobility. These warhorses, sometimes referred to as "Nisean chargers", were highly sought after by the Greeks, and are believed to have influenced many modern horse breeds. With the growing aggressiveness of cavalry in warfare, protection of the rider and the horse became paramount. This was especially true of peoples who treated cavalry as the basic arm of their military, such as the Ancient Persians, including the Medes and the successive Persian dynasties. To a larger extent, the same can be said of all the Ancient Iranian peoples: second only to perhaps the bow, horses were held in reverence and importance in these societies as their preferred and mastered medium of warfare, due to an intrinsic link throughout history with the domestication and evolution of the horse.