The Bastarnae provided the "casus belli" by crossing the Haemus and attacking the Dentheletae, a Thracian tribe who were Roman allies. Crassus marched to the Dentheletae's assistance, but the Bastarnae host hastily withdrew over the Haemus at his approach. Crassus followed them closely into Moesia but they would not be drawn into battle, withdrawing beyond the Tsibritsa. Crassus now turned his attention to the Moesi, his prime target. After a successful campaign which resulted in the submission of a substantial section of the Moesi, Crassus again sought out the Bastarnae. Discovering their location from some peace envoys they had sent to him, he lured them into battle near the Tsibritsa by a stratagem. Hiding his main body of troops in a wood, he stationed as bait a smaller vanguard in open ground before the wood. As expected, the Bastarnae attacked the vanguard in force, only to find themselves entangled in the full-scale pitched battle with the Romans that they had tried to avoid. The Bastarnae tried to retreat into the forest but were hampered by the wagon train carrying their women and children, as these could not move through the trees. Trapped into fighting to save their families, the Bastarnae were routed. Crassus personally killed their king, Deldo, in combat, a feat which qualified him for Rome's highest military honour, "spolia opima", but Augustus refused to award it on a technicality. Thousands of fleeing Bastarnae perished, many asphyxiated in nearby woods by encircling fires set by the Romans, others drowned trying to swim across the Danube. Nevertheless, a substantial force dug themselves into a powerful hillfort. Crassus laid siege to fort, but had to enlist the assistance of Rholes, a Getan petty king, to dislodge them, for which service Rholes was granted the title of "socius et amicus populi Romani" ("ally and friend of the Roman people").