Svatopluk I of Great Moravia intervened as an ally of Aribo because he had been at war with William and Engelschalk when the two died in 871.
The war between Arnulf and Svatopluk escalated after the former refused to surrender the Wilhelminers after a request.
After two and a half years, Charles himself came to Kaumberg to receive Svatopluk as his vassal and accept promises of peace.
The war is generally seen as having greatly augmented the power of Svatopluk I and strengthened his bonds to the Empire, as illustrating Charles diminished authority over his vassals and his inability to control their fidelity, and as rupturing the relationship between uncle and nephew and thus ruining the chances of Arnulf becoming Charles' heir or even receiving imperial largesse.
It is disputed whether the war really indicates the weakness or in fact the relative strength of the emperor or whether Arnulf ever had any potential of inheriting the imperial title in the first place.