By Billy Perrigo
November 26, 2018

Russia fired at and captured three Ukrainian boats in the sea off Crimea on Sunday, sparking a major escalation of tensions between the neighboring countries.

The news revived memories of Russia’s 2014 annexation of Crimea, the Black Sea peninsula previously controlled by Ukraine, and raised further concerns over Russia’s territorial ambitions.

At the center of the dispute is the strategically important Sea of Azov, a body of water bordered by Ukraine, Crimea and Russia and connected to the Black Sea by a strait to the south.

Here’s what to know about the situation.

What led to this?

In 2014, Ukraine erupted in a revolution that pitted pro-Russian and pro-E.U. segments of its population against one another, reflecting internal disputes over whether Ukraine should be closer with the nuclear superpower to its east or the democratic union to its west.

Not long after, Russian soldiers crossed the border into Crimea, the peninsula in the Black Sea that became part of Ukraine after the dissolution of the Soviet Union.

Against that backdrop, pro-Russian protests in eastern regions of Ukraine turned violent. International observers said disguised Russian forces were engaged in supporting the rebels. Russia denied involvement.

War in eastern Ukraine has continued ever since, leading to over 10,000 deaths and 1.7 million people being displaced. And while NATO and western governments condemned Russia’s annexation of Crimea, they did little to stop it.

What happened on Sunday?

The hostilities began when Russia blocked two Ukrainian military boats and one tugboat from sailing under a new Russian bridge spanning the Kerch strait, the channel that connects the Sea of Azov with the Black Sea to the south.

An oil tanker was placed under the bridge to block the Ukrainian boats’ access. A Russian coast guard vessel rammed the tugboat, and then Russian forces fired on all three boats, eventually capturing them. Six Ukrainians were injured, two seriously, while 23 were captured. Russian helicopters and warplanes were also involved in the clash.

Russia said the Ukrainian boats were illegally trying to enter its waters; Ukraine said they were abiding by international maritime law and a 2003 treaty that states both the strait and the Sea of Azov are shared territorial waters.

Petro Poroshenko, the Ukrainian President, responded by convening a war cabinet and tabling a vote on whether to implement martial law, which would have the effect of suspending elections scheduled for early next year that he is expected to lose.

On Sunday and Monday, NATO and western countries issued further condemnations of Russia over the hostilities at the bridge. The U.N. Security Council has called an emergency meeting over the crisis.

What’s changed since 2014?

When Ukraine controlled Crimea, it had one side of the Kerch strait, while Russia had the other. Following Russia’s annexation of Crimea, it now controls both sides.

In May this year, Russian President Vladimir Putin officially opened a $3.7 billion bridge across the strait, from the Russian mainland to Crimea.

And in recent months, Russia has begun inspecting Ukrainian vessels entering or departing Ukrainian ports in the Sea of Azov, despite the treaties that declare the waters to be shared territory.

With Ukrainian elections approaching next March, Russia might have chosen to intervene now to foment instability. Ukrainian lawmakers are set to vote later on Monday whether to implement martial law, which could give ammunition to Poroshenko’s political opponents.

Write to Billy Perrigo at billy.perrigo@time.com.

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