Russia has agreed to allow Iran to use the Volga-Don Canal so that Iranian ships can reach the Black Sea from the landlocked Caspian Sea. This is the first time Russia has ever given a foreign nation free access to the canal. Russia and Iran are now using each other’s Caspian Sea ports heavily for trade and getting Iranian weapons to Russia. Both nations have agreed to establish a joint-shipbuilding operation in the Caspian Sea and cooperate in dredging the canal, something that has not been done since 1991. The prolonged lack of dredging has made portions of the canal shallower and forced ships to carry less cargo. The 13 locks on the canal connect the Volga River, the longest in Russia that empties into the Caspian, and the Don River which empties into the Sea of Azov, which is connected to the Black Sea via the Kerch Strait.
The Caspian is the world's largest lake and it is huge, at 371,000 square kilometers (about the same size as Poland). It is about a thousand kilometers long and 430 kilometers wide. It's saline but is only about a third as salty as ocean water. The Caspian has a 7,000-kilometer-long coastline, with the largest chunk (1,900 kilometers) belonging to Kazakhstan.
Since 1952, a 101 kilometers long canal, linking the Don and Volga rivers, gave the Caspian Sea access to the Black Sea and the world's oceans. However, the largest ships that can use the canal cannot displace more than 5,000 tons and be no more than 140 meters (434 feet) long, 17 meters (52 feet) wide, and have a draft of no more than 3.5 meters (10.8 feet). The canal currently moves over 12 million tons of cargo a year. About half of that is oil or oil products.
The Caspian Sea, long a Russian lake, has undergone many changes since the Soviet Union broke up in 1991. Until then the only nations bordering the Caspian were Russia (with most of the coastline) and Iran. But after 1991 those two nations were joined by parts of the Soviet Union, Kazakhstan, Azerbaijan, and Turkmenistan, that became independent states.
Iran shares a land border with Azerbaijan and has a historical claim on Azerbaijan. In the 19th century Azerbaijan (as in the area occupied by the Azeris, a Turkic people) was divided by Russia and Iran. Currently, about a quarter of the Iranian population is Azeri, but the Azeris of Azerbaijan believe all Azeris should be part of an independent Azerbaijan. This was how it was for centuries before Turkey, Russia, and Iran began seeking to conquer Azerbaijan. Some Iranian Azeris like this idea and Iran is always looking for ways to make Azerbaijan back off. Iran has been expanding its Caspian naval forces, which annoyed Russia more than Azerbaijan.
The Iranian buildup included a new corvette, an Iranian built 1,400-ton ship. Azerbaijan responded by buying over $2 billion worth of weapons from Israel, which angered Iran a great deal. Among the items purchased were Gabriel anti-ship missiles. These are 522 kg (1,150 pound) weapons with a range of 36 kilometers. Azerbaijan uses these to protect its Caspian Sea coast from the growing number of Iranian warships being introduced in the Caspian. Most of the Iranian Caspian “fleet” consists of small patrol boats. Some are armed with anti-ship missiles but they are basically coast guard type craft.
What really controls the Caspian is aircraft and Russia has the most of those. Russia also has the only water link to the ocean and thus the ability to bring in more warships on short notice. These, plus Russia’s larger air force, gives Russia naval superiority in the Caspian Sea.
This is not enough for the nations that used to be part of the Soviet Union and are still on good terms with Russia. That’s because Iran has threatened all its neighbors on the Caspian and has claims on offshore oil fields that belong to Azerbaijan. There’s believed to be another 40 billion barrels of oil under the Caspian, and Iran wants to grab all it can. This makes all the other Caspian nations just a wee bit nervous.
Since the 19th century a Russian (and later Soviet) flotilla was the largest naval force in the Caspian. After the Soviet Union dissolved in 1991, Kazakhstan, Azerbaijan, and Turkmenistan inherited most of that. Although all Caspian nations have, sort of, pledged not to get involved in a naval arms race, Iran has broken that arrangement and everyone else responded by bringing more warships into the Caspian. Russia and its three allies have an advantage because they can buy from anywhere and bring the ships in via the Volga-Don Canal. Iran must build larger ships in the Caspian Sea yards. Smaller vessels can be brought in via railroad flatcars, which is what Iran does.
The new Russia-Iran alliance makes Kazakhstan, Azerbaijan, and Turkmenistan nervous because they are, after the Ukraine, according to Vladimir Putin, on the list of former Soviet territories that need to be reunited with Greater Russia. That would be difficult because these three states have growing economic ties with China, which has told Russia that the Ukraine War was counterproductive and bad for the Russian economy and armed forces. Putin refused to heed that advice from China and similar criticism from India. The Central Asian states oppose the war in Ukraine and agree with China and India.
Russia also angered the Central Asian states by forcibly or fraudulently getting Central Asian men in Russia to work to join the army. These men are not Russian citizens but Russia will accept alleged foreign Russian-speaking “volunteers” for the army. In practice they only need to be slow enough for Russian conscription teams to catch them. These Central Asian workers are often those who lost their jobs because of the severe economic sanctions the West inflicted on Russia for the Ukraine invasion. Many of these unemployed men are destitute and cannot even afford to get home. The Russians offer large cash bonuses for those to volunteer for up to a year of army service in Ukraine. The foreign workers don’t realize how dangerous that is. The danger to Russian diplomatic and economic relations was greater still.