Russian patrol ships opened fire on 2 armed Ukranian ships and a tugboat on Sunday near the Kerch Strait, wounding three sailors before seizing the ships. They released the ships a few hours later after blockading the strait.

The strait connects the Black Sea with the Azov Sea and passage through the narrow waterway has been disputed by Russia since they annexed the Crimea in 2014.  Ukraine has several vital ports on the Azov Sea and Russian control of the Kerch Strait means they can deny Ukranian ships passage from the Black Sea to their home ports on the Azov anytime they choose.

Politico:

Russia has reopened the Kerch Strait, which links the Azov Sea with the Black Sea, to civilian shipping today, the country’s TASS news agency reported.

The move came after Russia on Sunday opened fire on two Ukrainian armored artillery vessels and a tug boat off the coast of annexed Crimea, which Russia’s FSB security service claimed had illegally entered its territorial waters. Russian authorities then seized the three naval ships and blocked the Kerch Strait.

Ukrainian President Petro Poroshenko denied the vessels had done anything wrong.

The director-general of Crimean Maritime Ports, Alexei Volkov, told TASS that permission for civilian vessels to resume traveling through the channel was issued at about 4 a.m. this morning. Ships have already started moving through the strait, he said.

Several Ukrainian sailors suffered injuries during Sunday’s confrontation, according to the FSB and Ukraine.

In response to the incident, Poroshenko on Sunday asked parliament to declare martial law, which would limit civil liberties and give the state greater power. Lawmakers will consider the measures today.

Both Ukraine and Russia have requested an urgent U.N. Security Council meeting today on the incident.

This is Putin flexing his muscles at the expense of Ukraine. Russia fired on the Ukrainian ships and blockaded the straits simply because they had the ability to do so. And Ukraine can do little about it except protest to the UN.

There is also some gamesmanship at work. Putin has been stymied in his efforts to annex large parts of Eastern Ukraine where Russian speakers dominate. Despite denials by Putin, Russian troops are still deployed there, and violence occassionally breaks out between the Ukrianian army and Ukrainian separatists - armed and trained by Russia. 

Putting pressure on the Ukrainian government can be viewed as part of Putin's long game in Ukraine and his ultimate goal of re-establishing the old Soviet/Czarist empire.