and unify Northern India. Most of Southern Asia was unified under his grandson Ashoka the Great after the Kalinga War, though the empire collapsed not long after his reign. In China, the Shang dynasty and Zhou dynasty had risen and collapsed. This led to a Warring States period, in which several states continued to fight with each other over territory. Philosopher-strategists such as Confucius and Sun Tzu wrote various manuscripts on ancient warfare (as well as international diplomacy). The Warring States era philosopher Mozi (Micius) and his Mohist followers invented various siege weapons and siegecraft, including the Cloud Ladder (a four-wheeled, extendable ramp) to scale fortified walls during a siege of an enemy city. The warring states were first unified by Qin Shi Huang after a series of military conquests, creating the first empire in China. His empire was succeeded by the Han dynasty, which expanded into Central Asia, Northern China/Manchuria, Southern China, and present day Korea and Vietnam. The Han came into conflict with settled people such as the Wiman Joseon, and proto-Vietnamese Nanyue. They also came into conflict with the Xiongnu (Huns), Yuezhi, and other steppe civilizations. The Han defeated and drove the Xiongnus west, securing the city-states along the silk route that continued into the Parthian Empire. After the decline of central imperial authority, the Han Dynasty collapsed into an era of civil war and continuous warfare during the Three Kingdoms period in the 3rd century AD. The Achaemenid Persian Empire was founded by Cyrus the Great after conquering the Median Empire, Neo-Babylonian Empire, Lydia and Asia Minor. His successor Cambyses went on to conquer the Egyptian Empire, much of Central Asia, and parts of Greece, India and Libya. The empire later fell to Alexander the Great after defeating Darius III. After being ruled by the Seleucid dynasty, the Persian Empire was subsequently ruled by the Parthian and Sassanid dynasties, which were the Roman Empire's greatest rivals during the Roman-Persian Wars. In Greece, several city-states rose to power, including Athens and Sparta. The Greeks successfully stopped two Persian invasions, the first at the Battle of Marathon, where the Persians were led by Darius the Great, and the second at the Battle of Salamis, a naval battle where the Greek ships were deployed by orders of Themistocles and the Persians were under Xerxes I, and the land engagement of the Battle of Plataea. The Peloponnesian War then erupted