Russian-Iranian relations

How will Russia and Iran respond to the US challenge in the Caspian Sea?

On May 5 2018 Kazakhstan ratified an agreement with the US for the transit of US military cargo and possibly even troops (under the euphemism of ‘advisors’ or ‘engineers’) to Afghanistan via the Caspian Sea route.
The ostensible reason is that the US’s long established supply lines to Afghanistan via Pakistan are now ‘under threat’.  However, as is often the case nowadays, the reality is rather more complicated.