In 2014, Russia had annexed the nearby Crimean Peninsula, which is predominantly internationally recognised as Ukrainian territory. It later constructed the Crimean Bridge across the strait. Under a 2003 treaty, the strait and the Azov Sea are intended to be the shared territorial waters of both countries, and freely accessible.[9][13][14] Russia, in turn, insists that, while the 2003 treaty remains legally valid, Ukrainian ships must ask for permission before entering Russian waters along the perimeter of Crimea as any transnational water crossing, as regulated by the Law of the Sea Treaty.[15] The annexation of Crimea by Russia is recognized only by Russia and a small number of its allies, therefore Ukraine holds that the invocation of an international treaty with regard to Crimean waters is illegitimate.[16]
As the flotilla, which consisted of two gunboats and a tugboat, approached the Kerch Strait, the Russian coast guard said they repeatedly asked the Ukrainian vessels to leave what they referred to as "Russian territorial waters". They said that the vessels had not followed the formal procedure for passage through the strait, that the Ukrainian ships had been manoeuvring dangerously, and that they were not responding to radio communications.[9][17][18] Ukraine said that it had given advance notice to the Russians that the vessels would be moving through the strait, that the ships had made radio contact with the Russians, but received no response, and cited the 2003 treaty against the assertion that the ships had entered Russian territorial waters.[19][20][21] The Russians tried to halt the Ukrainian ships, but they continued moving in the direction of the bridge. As they neared the bridge, the Russian authorities placed a large cargo ship under it, blocking passage into the Azov Sea. The Ukrainian ships remained moored in the strait for eight hours, before turning back to return to port in Odesa. The Russian coast guard pursued them as they left the area, and later fired upon and seized the vessels in international waters off the coast of Crimea.[17][22][9][23][24] Three Ukrainian crew members were injured in the clash, and all twenty-four Ukrainian sailors from the captured ships were detained by Russia.[9][8][25]
The Ukrainian president characterised the incident as a potential precursor to a Russian invasion, and declared martial law along the border with Russia and in Black Sea coastal areas, which expired on 26 December 2018.[26][27] The Russian government called the incident a deliberate provocation by Ukrainian President Petro Poroshenko ahead of the 2019 Ukrainian presidential election.[28] The incident took place a few days before the 2018 G20 Buenos Aires summit. Western leaders referred to it when they spoke of sanctions against Russia.