Antony was a fine commander, but he was no Caesar. Had the latter lived to carry out the invasion of Parthia, it probably would have ended in Roman victory.
Trajan, while failing at Hatra, had done much to at least stabilize the Roman hold on Mesopotamia, chiefly through the efforts of his great lieutenant Lucius Quietus.
Its a shame that the sources for Trjan's Parthian War are so poor, but I must say that I am a bit puzzled by the assertions of some modern historians that Trajan left the area a broken, defeated man. I don't think such a man would have been planning to resume the war in 117 had he not felt that victory was in his grasp. Only his rapidly declining health prevented this new campaign, and he probably died thinking that Hadrian would honor his wishes and complete the war (if indeed he chose Hadrian to succeed him that is).
The Romans were lucky that the Parthians lacked the strength to seriously capitalize on the defeats of Crassus and Antony, while the Parthians were lucky that perhaps the greatest Roman general of all -- Caesar -- was assassinated before he could avenge Crassus, and that perhaps the most powerful emperor of all -- Trajan -- began his campaign as a relatively old man. They were also lucky that the plague ravaged the victorious army of Cassius Avidius, who had won a resounding victory on behalf of Marcus Aurelius in the year 165, thus possibly saving the Parthians from a full-scale invasion of Media.
I wish the Romans had thought the entire Arabian peninsula (well, the coasts) worth conquering. Mohammed's ancestors (one or more of them) might have gotten whacked, and that would have had a positive impact throughout history.
"Antony was a fine commander, but he was no Caesar."
Quite agree. He was a real combat leader, but took direction more capably than giving it, IMO. And of course, it didn't help that he was more and more just a drunk. He had less political sense than Octavian, and remained far from Rome most of the time. Had he succeeded in Parthia, Octavian might have been a minor footnote today. :') Of course, it's just as likely that, had Antony succeeded in Parthia, he would have continued to unravel, Octavian would have been unwilling to relinquish power, continued to scheme, war would have come anyway...