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First Dacian war

When Trajan came to power in 98, he immediately toured the Danube area and ordered the strengthening of fortifications along the Dacian frontier. Three years later, Trajan decided to launch an offensive against Dacia. According to Cassius Dio this was because "he had taken stock of previous record, resented the annual sums of money they were getting, and saw that their powers and their pride were on the increase." Trajan's force crossed the Danube in 101 and advanced into Dacia, pushing back Dacian forces. According to Dio, Decebalus sent envoys asking for negotiations, but Trajan refused a personal meeting.

At the Second Battle of Tapae, Decebalus was defeated, but he inflicted serious losses on the Romans. Trajan chose not to pursue the war until the spring. Decebalus tried to wrongfoot Trajan by launching a surprise attack on Moesia, but he suffered a major defeat at the Battle of Adamclisi. Despite stiff resistance, the Romans closed on the Dacian capital by early 102. Decebalus was forced to concede defeat and accept Trajan's terms, which included the loss of some territories in the vicinity of the Danube and the dismantling of his fortresses. However, Decebalus retained his throne.