How did Trajan’s reign help bring the Roman Empire to its greatest territorial extent?

Trajan’s reign was pivotal in expanding the Roman Empire to its greatest territorial extent because he abandoned the long-standing policy, established by Augustus, of maintaining static frontiers. Instead, he pursued an aggressive military strategy that incorporated several new regions into the empire.1234

His expansionist efforts included:

• Dacia: Following two hard-fought wars (101–102 and 105–106) against Decebalus, Trajan conquered and annexed Dacia. This created a new province north of the Danube, providing land for settlers and access to valuable gold and salt mines.15
• Arabia: In 105–106, the Nabataean kingdom was annexed, becoming the Roman province of Arabia.14
• The East: Trajan campaigned against the Parthians, Rome's traditional eastern rival. He incorporated Armenia and, in 115–116, annexed upper Mesopotamia and Assyria, eventually reaching the Persian Gulf.142

While these campaigns achieved unprecedented territorial growth, the rapid expansion proved difficult to consolidate, leading to widespread revolts in the Middle East toward the end of his reign.4