Short answers, pulled from the story.
Seleucus I Nicator established the Seleucid Empire in 312 BC after receiving Babylonia and Assyria from Perdiccas. This date marked the beginning of an empire that stretched across West Asia for nearly two centuries.
The Seleucid Empire encompassed Anatolia, Persia, the Levant, parts of Turkmenistan, modern-day Iraq, Iran, Afghanistan, Syria, and Lebanon. The domain stretched from the Aegean Sea to Afghanistan Pakistan including diverse array cultures ethnic groups Greeks Assyrians Armenians Georgians Persians Medes Mesopotamians Jews living within bounds.
Antiochus III the Great lost control over western Anatolia after suffering resounding defeats against Rome and its Greek allies at battles of Thermopylae in 191 BC and Magnesia in 190 BC. The Treaty of Apamea signed in 188 BC required the Seleucids to pay large indemnity, retreat from Anatolia, and never again attempt expansion west of Taurus Mountains.
Pompey set about task remaking Hellenistic East creating new client kingdoms establishing provinces once Mithridates defeated by Pompey in 63 BC. Pompey saw Seleucids too troublesome continue doing away both rival Seleucid princes making Syria Roman province.
The Seleucid army fought primarily Greco-Macedonian style main body phalanx large dense formation men armed small shields long pike called sarissa developed Macedonian army reign Philip II Macedon his son Alexander Great. Size Seleucid army usually varied between 70,000 and 200,000 manpower alongside numerous native mercenary troops supplement Greek forces limited due distance from rulers' Macedonian homeland.