Short answers, pulled from the story.
The Seleucid Empire began in 312 BC when Seleucus I Nicator arrived in Babylon and ended in 63 BC when Pompey made Syria a Roman province. Historians use 312 BC as the foundation date for the Hellenistic era in the East and 63 BC as the year of its ultimate overthrow.
Seleucus I Nicator founded the Seleucid Empire in 312 BC, creating a domain that stretched from the Aegean Sea to the borders of India. The empire's greatest extent included Anatolia, Persia, the Levant, Mesopotamia, and modern-day Kuwait, Afghanistan, and parts of Turkmenistan.
The conflict between Seleucus I and Chandragupta Maurya in 305 BC resulted in a political alliance and the cession of territory west of the Indus River. Chandragupta received Seleucus's daughter as a gift while Seleucus received 500 war elephants that played a decisive role at the Battle of Ipsus in 301 BC.
The Seleucid Empire ended in 63 BC when the Roman general Pompey overthrew the last Seleucid princes and made Syria a Roman province. The empire was previously reduced to a rump state in Syria after its conquest by Tigranes the Great of Armenia in 83 BC.
The Circle in the Sand incident occurred in 168 BC when Roman Proconsul Gaius Popillius Laenas drew a circle in the sand around Antiochus IV Epiphanes and demanded an immediate reply to a decree. The king chose to withdraw from his attack on Alexandria rather than set the empire to war with Rome again.
The Seleucid Empire maintained its military by paying armies exclusively in silver and adopted the Attic Standard in coastal regions while Mesopotamia used the shekel. Agriculture constituted 80 to 90 percent of the population, and the state managed irrigation and waterways to ensure grain production and economic stability.