Istanbul Kanal, the planned 45-km artificial waterway connecting the Black Sea and the Sea of Marmara, is probably Turkish President Recep Tayyip Erdogan's most ambitious  large-scale infrastructure project since he took office 18 years ago — and also the one that will bring the most everlasting consequences, both globally and domestically.
First unveiled in 2011 , Turkey's “second Bosphorus”, as it's been called, received the green light from the Environment Ministry only this past January. Erdogan justifies the $12.6bn-project to relieve ship traffic in the Bosphorus strait. The new waterway will have a capacity of around 160 vessel crossings  per day, according to authorities. For comparison, the Suez Canal supports 50 crossings  a day, and the Panama Canal, around 50 .
But experts say that, if built, the new canal could make the 1936 Montreaux Convention, which regulates the straits of Dardanelles and Bosphorus, obsolete.
Besides granting Turkey full sovereignty over both straits and ensuring passage of civilian vessels during peacetime, the treaty most importantly limits military deployments  in the Black Sea to 21 days for states not bordering that body of water, which is effectively dominated by Russia.
President Erdogan has said multiple times  that the convention will not apply to the new waterway. In practice, it means that Turkey, a NATO member, will be able to allow any military ships into the Black Sea — including those with United States flags.
The Istanbul Kanal, however, only provides an alternative route to the Bosphorus; ships would still have to use the Dardanelles strait, which connects the Sea of Marmara and the Mediterranean and is covered by the Montreaux Convention. Because of that, it has been widely speculated that Turkey might also withdraw from the Convention altogether, something officials have so far denied .
Earlier this month, such speculations have led a group of Turkish former admirals to publish  a statement warning of the dangers of a possible withdrawal. “Montreux has allowed Turkey to stay neutral during World War II,” the statement said. “We are against any kinds of talks and actions which may result in Montreux being open for debate.”
The statement produced a political firestorm in Turkey, with Ankara equating it with an “attempted coup.”  Hundreds of civil society organizations, presumably all pro-government, filed criminal complaints  against the former admirals, and 14 were detained  (but released shortly after ).
Turkey's civil authorities have a fraught relationship with the military, as the latter has staged military coups in 1960 and 1980  — and a failed attempt in 2016.
But control over the Bosphorus and the Dardanelles straits goes way further into the past. In 1774, with the Treaty of Küçük Kaynarjai, the Ottomans forbade all foreign ships from sailing out of the Black Sea, which resulted in Russia being unable to send its Black Sea fleet to Japan  in the war of 1904-1905.
In 1841, the London Straits Convention forbade non-Turkish warships to enter the straits during peacetime. This treaty was replaced by the Lausanne Treaty in 1923, which called for the demilitarization of the Dardanelles and several other islands , as well as opened the Aegean Sea for merchant ships.
Then came the Montreaux Convention, which, according to historian Onur Isci in an interview  with Al-Monitor, “was negotiated as Hitler’s shadow loomed ever larger. It struck what seemed a near-impossible balance, neither alienating Russia nor the Allied powers while securing maximum gains for Turkey.”
Writing for Carnegie Europe  about the geopolitical Pandora's box that the new waterway could open, Marc Pierini says:
If the convention will not apply to the canal, it would in practice mean nullifying it and unilaterally creating a new role (and new rights?) for Turkey in maritime traffic regulation between the two seas. Questions abound. Would Turkey set different rules for maritime traffic on the new canal compared to the convention applying to the Bosporus? Would it—in the absence of any international authority or treaty—be free to open or close transit through the canal to certain flags at its sole discretion?
There will be domestic consequences too.
Experts say  that, because of a 50 cm level difference between the Sea of Marmara and the Black Sea, the connection will impact on both sea's salinity, and in the lives of organisms living in both seas. Furthermore, the canal will pass through Küçükçekmece lagoon, which is an important stop for migratory birds.
In an interview with The Guardian, Istanbul secretary of the Union of Chambers of Turkish Engineers and Architects, Cevahir Efe Akçelik, said :
The salinity of Black Sea is less than Marmara Sea, and the organic content of Black Sea is much higher than that of Marmara Sea. Some oceanographers say 30 years later there will be no oxygen left in the Marmara Sea. It’s a really harmful and dangerous project.
The canal could also pose a threat to Terkos Lake and Sazlıdere, which supplies drinking water to Istanbul.
Akçelik told The Guardian  that “should those reserves be lost, there is no alternative water source on the European side of Istanbul. Instead, the government would have to pump water from the Sakarya River, deep on the Asian side.”
According to the results of one survey  by the Istanbul Mayorship from August 2020, more than 60 percent of Istanbul residents opposed the construction of the canal.