"To what extent could this operation in Basra be seen as an American attempt to secure their supply lines in view of a forthcoming attack on Iran?"

One of SicSemperTyrannis' readers "Pale Rider" offers this

"....The supply lines that run from Camp Doha in Kuwait into Iraq are pretty tenuous, as all supply lines are. We would have to detail some significant forces to keep them open if the elements in Basra that are opposing us decided to try to shut them down......The majority of their gear is fitted for COIN not ground assault. Much of it is worn out.......A ground assault into Iran would have to be organized around the division and corps formations that aren't really in place in Iraq.......Petraeus would literally have to start this week in order to get even 50-60,000 troops in place to attack Iran......"