Seleucus I had immense territorial ambitions and extended his empire east to cover central Asia Minor, Mesopotamia, parts of Persia, Afghanistan, and Pakistan. He ruled over much of Alexander's eastern empire reaching up to the Indus River. In 301 BC, he conquered Syria, followed by Anatolia in 281 BC. The Seleucid Empire was essentially Greek-Macedonian, and Seleucus conquests did much to spread Hellenistic culture across Europe, Asia Minor, and in parts of Asia.
In 281 BC, Seleucus I was murdered, and his son, Antiochus I, succeeded him. In 261 BC, Antiochus II took over. These Seleucid emperors ruled a vast empire spanning from the Aegean Sea to the Indus River valley.
The Seleucid Monarchs
The spread of the Greek culture and language across the Seleucid Empire was partly due to the adoption of a Hellenistic court culture by the Seleucid rulers. The local languages were also used for administrative purposes. Seleucus established the two cities that were to be his capitals - Seleucia in Iraq and Antioch in Syria. Babylon remained the most important city and cultural hub of the empire.
Seleucus and his successors were excellent administrators. They built many beautiful structures in the cities modeled on the Greek polis. The Seleucid rulers used diplomacy as a tool to maintain peace and stability through their empire. They married into the Persian royal dynasties of Cappadocia and protected the Persian religion, thus winning the hearts of their subjects.
The Seleucid kings maintained cordial relations with the Mauryan kings of India. The dynasties had signed peace treaties and the Mauryans presented the Seleucid with many elephants.
Antiochus the Great
In 246 BC, the Bactria and the Parthia satraps rebelled and broke free. But Antiochus III (Antiochus the Great) won back these provinces and annexed Palestine in 200 BC and Thrace in 196 BC. Antiochus III soon came into conflict with the Romans. His entire army was routed in the Battle of Magnesia in 190 BC. With the death of Antiochus III in 187 BC, the Seleucid Empire was greatly weakened and the satraps of Bactria and Parthia soon declared their independence.
Fall of the Empire
The Seleucid defeat at the Battle of Magnesia meant the loss of most of Asia Minor. The war, paid for by the Seleucids, depleted the royal exchequer. Antiochus IV defiled the Temple of Jerusalem in 168 BC, perhaps in an attempt to seize the temple treasure. A loss of the goodwill built up by earlier Seleucid kings was compounded by the resistance put up by the Hasmonaeans. The Seleucids lost Palestine. The Parni and Armenian invasions eroded most of the Seleucid territory. The Romans took over Syria and Antioch by 64 BC, effectively ending the rule of the Seleucid dynasty.