Previously, Russia relied on two separate routes to supply and rotate its forces in Transdniestria: an overland route through Ukraine and airlifts to Chisinau in Moldova proper, from where Russian forces were allowed to cross into Transdniestria over land. The land route through Ukraine has been closed for more than a year, and on June 8, Ukraine formalized its withdrawal from the agreement with Russia that allowed the use of this land route. The Moldovan route is unreliable at best; Moldova has not recognized the breakaway state of Transdniestria and obviously has concerns about Russian forces there. Even though a small peacekeeping force including Moldovan, Transdniestrian and Russian forces is present in the breakaway territory, some troops in the Russian contingent have been turned away after arriving in Chisinau.
For now, these limitations on logistical support have not cut off the Russian forces in Transdniestria. A runway in Tiraspol, the capital of Transdniestria, allows Russia to establish an air bridge that can be used to move troops, equipment and supplies into the breakaway state. Although the runway is in bad shape, Russian passenger aircraft have used it in the last few years, showing that it can still receive robust military cargo aircraft.
Moreover, Russia's minimal presence in Transdniestria limits the amount of supplies needed. In terms of consumables and fuel supplies, the force of about 1,400 soldiers can rely mostly on locally available supplies. The main reasons Russia needs to keep a supply route open are to rotate — and, in the event of a crisis, reinforce — its forces in Transdniestria and to move military equipment.
The air bridge is not without contestation, either; in order to reach Transdniestria, Russian aircraft need to pass through foreign air space. The most direct route takes Russian cargo aircraft through Ukrainian air space over Odessa. A longer route would go through Romanian and Moldovan air space. This is why the potential deployment of the S-300 air defense systems in Ukraine's Odessa region matters for Russia. Although the potential cost of interdicting Russian flights would be incredibly high and would essentially constitute a declaration of war against Russia, the deployment of these systems establishes the capability to do so.
Even if Russia continues its airlift operations into Transdniestria, it will have to face the fact that its air route is inherently vulnerable and, in case of an outright conflict, would be lost easily. There is no immediate way for Russia to counter Ukraine and Moldova's ability to interdict its flights into Transdniestria, but this situation could lead Russia to decide to reinforce its limited position in Transdniestria to make it robust enough to withstand isolation or possibly assist in maintaining access. Such reinforcement, whether in terms of troop size or more advanced equipment, will of course further fuel the political standoff surrounding Transdniestria and the Russian presence there.
The alleged deployment of S-300s in Ukraine comes amid increasing political assertiveness by both Ukraine and Russia. The Russians view the recent appointment of former Georgian President Mikhail Saakashvili as the governor of Odessa and his announcement that reinforcing Ukraine's border with Transdniestria is a top priority as a clear provocation. In the meantime, there has been an apparent uptick in security exercises and military drills within Transdniestria, which is likely intended as a message from Russia that its security forces will be prepared in case of any conflict in the region. Actual military hostilities between Ukraine and Transdniestria remain unlikely. However, large security buildups and increased military activities from both sides increase the chance of miscalculation in a region that has gained significance in the standoff between Russia and the West.