Key maritime straits remain central to trade and geopolitical tensions

Maritime straits including Hormuz, Bab el-Mandeb, Malacca, the Bosphorus and the Dardanelles remain central to global trade and have repeatedly been linked to geopolitical rivalry.

The Strait of Hormuz is one focus of current tension as Iran tightens its grip there. Experts say Yemen's Houthi rebels, Iran's allies, want to disrupt Red Sea shipping to and from Saudi ports and could also affect traffic in Bab el-Mandeb, the passage ships use to reach the Suez Canal.

Hormuz and Bab el-Mandeb are among roughly a dozen straits considered vital to international trade. Oil and gas tankers, bulk carriers and container ships carrying raw materials and goods use these narrow natural passages.

Sylvain Domergue, a lecturer at Sciences Po Bordeaux and author of a book on the geopolitics of maritime spaces, said the United Nations Convention on the Law of the Sea, signed in Montego Bay on December 10, 1982, sets out that a strait is a natural feature not subject to tolls and that free navigation is guaranteed. He contrasted this with man-made canals such as Panama and Suez, where states that control them can charge tolls.

Domergue said Iran has not ratified the Montego Bay Convention but had until now respected it under customary law. He said the United States also has not ratified the convention, while respecting almost all of it.

The strategic importance of straits dates back centuries. Domergue said Portugal tried in 1507 to control Hormuz to secure access to spices and wealth from the East. In the early 17th century, the Mare Liberum doctrine associated with Dutch jurist and philosopher Hugo Grotius helped establish the idea that the sea belongs to no one and that freedom of navigation requires freedom of passage through straits.

Britain later built military positions around strategic waterways. Its naval base in Singapore gave it influence over Malacca, now used by a large share of maritime traffic from China. Britain also established bases on Socotra, near Bab el-Mandeb and the entrance to the Red Sea.

In Europe, Domergue said the Danish straits, including the Oresund and the Kattegat, were important in the 19th century because Denmark collected tolls there for a long period. He said they also remain strategic because they block Baltic powers, especially Russia, from access to the Atlantic.

The Turkish Straits, the Bosphorus and the Dardanelles, are described as the only major straits where a toll is still collected. They are governed by earlier agreements signed in Montreux in 1936.

Domergue said future issues could concern the Taiwan Strait, in light of China's territorial ambitions in Southeast Asia, and the Strait of Malacca, which is shared by Singapore, Malaysia and Indonesia. He said Chinese vessels have collected marine sand for construction in waters claimed by Taiwan, calling these actions provocative.

He also said that after Iran announced plans to introduce a toll in Hormuz, Malaysia's prime minister initially said it was a good idea that he might copy for Malacca, before stepping back from that position. Domergue said a toll in Malacca would hurt Singapore's status as a hub of East Asia.