BERDYANSK, Ukraine — Standing on the freezing deck of a Ukrainian coast guard patrol boat, looking out at the flat gray Sea of Azov, one can easily understand why Vladimir Putin believes he can bend Ukraine to his will.

While the world focuses on Putin’s massive military buildup around Ukraine’s land borders, far less attention has been paid to the Russian threat from the sea.

Six Russian amphibious landing ships are moving from the Mediterranean into the Black Sea to join Russia’s Black Sea fleet for “exercises” opposite Ukraine’s main naval port of Odessa. Meanwhile, Russian ships are monitoring Berdyansk and neighboring Mariupol, the two main Ukrainian ports on the Sea of Azov, a northern extension of the Black Sea.

Coast Guard Capt. Roman Varianitsyn, my host on the patrol boat, is painfully aware of Russia’s grim maritime challenge to his country, which he has been dealing with professionally and personally for the last eight years.

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“Our responsibility is to patrol the borders of the Azov,” he tells me, standing tall despite the low roof of the control room, in a blue uniform, blue wool cap, and windbreaker. “But after Russia illegally invaded and annexed Crimea they got to decide who goes in and goes out of this Azov sea.”

His coast guard base is located in this seaside resort town of Berdyansk, whose harbor and beaches curve into the Azov. The town looks peaceful on the surface. Its water park, gated summer camps, and small hotels have shuttered for the chilly winter. But only an hour up the coast is the front line, the crossing between government-controlled territory and the chunk of the Donbas region occupied by Russian proxies and Russian soldiers.

And off the coast of Berdyansk, the Russian navy has been harassing Ukrainian ships for nearly a decade.

Varianitsyn remembers well when the trouble started in 2014. That’s when Russian troops without insignia or uniforms suddenly invaded the Crimean peninsula, which straddles both the Black Sea and the Azov. Most of Ukraine’s navy was based on the peninsula, and he had to swiftly sail the navy ship he then commanded out to sea and away from the Russians, leaving his wife and 2-year-old daughter behind.

Only later could he return to evacuate his family in a civilian convoy.

The result of that invasion was a decimated Ukrainian navy that is only now rebuilding.

Having switched to the coast guard, the captain had a close call in September 2014 when his patrol boat was hit by an antitank missile fired by pro-Russian forces on the mainland. The boat exploded and two of his colleagues died.

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In the eight years since Russia annexed Crimea, the Russian navy has delighted in harassing and humiliating Ukrainian shipping, Varianitsyn tells me. Although both Russia and Ukraine have equal rights to use the Azov (as the only two nations that border the sea), the Russians have taken control of the Kerch Strait: the single narrow passage into the small sea.

“Russia completely overturned the rules and they get to decide who passes,” the captain relates bitterly.

In 2018, the Russian Black Sea fleet took a Ukrainian navy ship hostage. In 2019, a Russian naval ship tailed Varianitsyn’s coast guard boat, advancing directly on them. Loudspeakers warned that if they continued to pass without permission from Russia, they would be fired upon.

Russia also uses its control of the Kerch Strait to harm Ukraine’s economy. “They stall international commercial ships on the way to Mariupol and Berdyansk to pick up steel and grain for export,” the captain tells me. “This is a way to directly influence the economic situation in Ukraine.”

Had I been able to go five miles out to sea on Varianitsyn’s patrol boat, I would have seen “at least two to four Russian ships that have been actively monitoring the two Ukrainian ports on a regular basis since 2018.” Ukrainian naval and coast guard ships don’t even try to enter the Kerch Strait anymore.

But the captain’s patrol boat, nostalgically nicknamed the Arabat after a spit of land in Crimea, never set out to sea on the icy day I visited. The coast guard was wary, I was told, of using the Arabat to break heavy ice unless it was a military necessity. I wondered if coast guard officials were concerned that the boat, built in 1993, was showing its age. Or perhaps Varianitsyn was wary we would meet up with an aggressive Russian patrol boat.

I did join the daily coast guard shore patrol to check whether any unknown small boats had been approaching local beaches, but our binoculars spotted nothing but empty sea.

However, it didn’t take a sighting to grasp the essential message laid out so clearly by Varianitsyn on my visit to the Arabat. The Russian navy believes that Crimea, and indeed all Ukraine, is Russia’s. Moscow doesn’t feel the need to adhere to accords between the two countries on access to the Azov, or to respect international maritime rules.

Whether or not Vladimir Putin means his Black Sea exercises as a prelude to attack, Russia’s naval behavior is yet another example of the Russian leader’s willingness to use force when he believes he can get away with it.

However, Varianitsyn told me one story that carries a different message. When Russian warships held the Ukrainian naval vessel hostage in 2018, they finally released it after intense international pressure.

Only strong, unrelenting Western pushback will make Putin reconsider his blackmail of Ukraine at sea, as he is doing on land. Otherwise, when it comes to the Azov (and increasingly the Black Sea) he will keep trying to make Ukraine’s territory his own.