We have two different versions on the reasons Caracalla invaded Parthia, one from Cassius Dio and another from Herodian, the latter having the best known version. According to Dio, Caracalla offered his hand in marriage to the daughter of Artabanus in order to seal an alliance between the Roman and Parthian Empires, but he was refused. Caracalla took this as a pretext to launch an invasion into Parthia, and war swiftly began. Herodian repeats the offer of alliance via marriage, and adds that Caracalla claimed that the united strength of both empires would be utterly unstoppable, and that if combined as one their formidable armies could complement one another: Rome with its infantry, Parthia with its cavalry. He also added that he didn't wish to be married to some Roman noble, but the daughter of a great king instead. Artabanus was sceptical and for good reason: this was an unprecedented offer from Rome, and the king at first replied trying to politely decline. However Caracalla nagged at Artabanus constantly and sent him lavish gifts and oaths of friendship, and thus finally won him over and had the marriage arranged to take place in Mesopotamia.
There deffinetly was a marriage at least discussed of between Rome and Parthia, and Caracalla did deffinetly invade as a result of this arrangement regardless of which version is more true. Either way Caracalla had no intention of keeping his side of the bargain, and had only planned out the marriage to ensure he was warmly received into Parthia without opposition, granting him the element of surprise: crude, but good strategy. Dio and Herodian once again disagree on another, factor: Artabanus' realization of the impending Roman invasion. Dio claims that Artabanus did not believe Caracalla's offer was genuine, or at least so its implied, and thus rejected it. Herodian claims that, while perhaps suspicious at the beginning, Artabanus eventually let his guard down completely and even addressed Caracalla as his son-in-law in letters between the two. Whatever the reality, Caracalla deffinetly did invade as a result of this marriage deal, and Artabanus was very unprepared for an invasion either way.