And the difference--a most momentous one--was this.

They besieged and captured the place after defeating the greatest force that Parthia could bring into the field against them.
Julian found himself in front of Ctesiphon before he had crossed swords with the Persian king, or so much as set eyes on the grand army which Sapor was known to have collected.

To have sat down before Ctesiphon under such circumstances would have been to expose himself to great peril; while he was intent upon the siege, he might at any time have been attacked by a relieving army under the Great King, have been placed between two fires, and compelled to engage at extreme disadvantage.

It was a consideration of this danger that impelled the council of war, whereto he submitted the question, to pronounce the siege of Ctesiphon too hazardous an operation, and to dissuade the emperor from attempting it.
But, if the city were not to be besieged, what course could with any prudence be adopted?
It would have been madness to leave Ctesiphon unassailed, and to press forward against Susa and Persepolis.

It would have been futile to remain encamped before the walls without commencing a siege.