Russia annexed Crimea in 2014. The annexation is not officially recognised by the United Nations.
The Kerch Strait connects the Sea of Azov with the Black Sea, and is formed by the coasts of the Russian Taman Peninsula and disputed Crimea. It is the sole access point for ships travelling to and from Ukraine's eastern port cities, most notably Mariupol. While both Ukraine and Russia agreed to the principle of freedom of movement through the strait and the Sea of Azov in 2003 following the Tuzla Island incident, Russia has controlled both sides of the strait since the Crimean annexation. By May 2018, Russia had completed the construction of the Crimean Bridge, which is 19 kilometres (12 mi) long and spans the strait, providing a direct land connection between Crimea and the Taman Peninsula. The bridge's construction was criticised by Ukraine and other countries, which called it illegal. Furthermore, the Ukrainian and American governments have said that the bridge is being used by Russia as part of a creeping hybrid blockade of Ukrainian ports in the Azov Sea, and that Russian inspections of ships have risen sharply since the bridge opened in May 2018, with some reportedly being forced to wait between three and seven days before being allowed through. Under the 2003 treaty, both Russia and Ukraine have the right to inspect vessels sailing into or out of the Sea of Azov. Ukraine has said that the increase in inspections by the Russian coast guard following the opening of the bridge represents an abuse of that right.
According to the Defense News, "From Russia’s perspective, tensions began flaring in March, when Ukrainian coast guard vessels in the Sea of Azov seized the Nord, a Russian-flagged fishing boat operating out of the Crimean city of Kerch." In March 2018, the Ukrainian border guards detained in the Sea of Azov fishing vessel Nord, accusing the crew of entering "territory, which has been under a temporary occupation". The captain of the Nord, Vladimir Gorbenko, is facing up to five years in prison.
In late September, the Ukrainian Navy launched an operation to move the Donbas search-and-rescue ship and the
Korets tugboat from Odessa to Mariupol. The operation was the first deployment of Ukrainian Navy ships to the Kerch Strait area since the Russian annexation of Crimea. The vessels proceeded from Odesa with the 48-yr old Donbas towing the 45-yr old Korets. Commanded by Dmytro Kovalenko, Ukrainian Naval Forces Deputy Chief of Staff, the ships radioed their intention to enter the Azov Sea via the Kerch Strait as they approached it on 23 September, but did not follow the official procedure to request permission. According to Kovalenko, this was an intentional form of "naval diplomacy", carried out with the aim of asserting the Ukrainian claim to the surrounding waters. While the ships received pilot services from the Kerch port authority free of charge, they were also tailed by at least 13 Russian vessels, and flown over by Russian aircraft. Ultimately, the Ukrainian vessels complied with transit procedures which did not require a request for permission to transit, Russia did not hinder the ships' passage under the Crimean Bridge, and they successfully reached Mariupol. In an interview with the Kyiv Post, Ukrainian naval expert
Taras Chmut said that he thought that Russians had not expected the Ukrainian operation, and so decided to take the least risky option by allowing them through. He also said "For the first time, we didn’t just react to the Russians’ steps, but started to set our own game rules".
EU Commissioner for Security Sir Julian King said that Russia had staged a year-long disinformation campaign in order to "soften up" public opinion in preparation for the incident. According to King, numerous rumours were spread about the plans of the Ukrainian authorities, including that the Ukrainian government had begun dredging the Azov Sea in preparation for the arrival of a NATO fleet, that it intended to infect the Black Sea with cholera, and that it planned to blow up the Crimean Bridge with a nuclear bomb.
Pavel Felgenhauer, a Moscow-based defense analyst and columnist for Novaya Gazeta, speculated that Putin's government instigated the incident out of concern that Ukraine’s naval bases in the Sea of Azov may eventually host visiting NATO patrols.