Ukraine's uneasy ties with Russia are symbolised by a long-running dispute in Crimea. The BBC's Gabriel Gatehouse reports from Ukraine's southern region on how Kiev's ambition to join Nato is affecting the two neighbours.
Uneasy neighbours: Russian and Ukrainian ships docked together
Take a trip across Sevastopol harbour, and you will see Ukrainian battleships at anchor side-by-side with those of the Russian Black Sea Fleet: two navies next to each other, like long-time but somewhat uneasy neighbours.
The prospect of Ukraine joining Nato may still be a decade or more away, but it worries the Russian defence ministry.
Since the end of the Cold War, Nato has expanded right up to Russia's borders, taking in countries such as the Baltic states that were formerly part of the Soviet Union.
But for Russia, Ukraine is a special case, not least because of the presence of the Black Sea Fleet in Ukrainian waters.
On board the Ukrainian navy's flagship, the Hetman Sahaidachnyi, Vice Admiral Ihor Tenyukh talks a lot about the bond all sailors feel, regardless of the country they serve. But when it comes to Russia's fears about Nato, he is unmoved.
Vice Admiral Tenyukh looks to the West, like many Ukrainians
"It's not my job to reassure or persuade, that's the job of other government departments," he says.
"I think every country must decide on its own direction. Ukraine decided long ago to integrate with Europe. These decisions are perfectly logical, and Nato is an important step on the way to membership of the European Union."
In the aftermath of the collapse of the USSR, Moscow struck a deal with Kiev: the Russian Black Sea fleet could stay in a port that now belonged to Ukraine.
That deal, though, came at a price - some $100m a year, and the political uncertainty of having to rely on the goodwill of a foreign government.
And for many Ukrainians, like local MP Volodymyr Arabadzhy, the presence of the Russian fleet creates a stark dilemma.
"If we go towards Nato, the Russian fleet will have to leave Sevastopol. If the Russian Fleet is in Sevastopol, Ukraine cannot be in Nato," he says.
Like many Crimeans, he feels a strong attachment to the Black Sea Fleet.
"Our people in Sevastopol have many traditions - mostly close to the navy fleet, because Sevastopol was born as a naval base 225 years ago."
The Russian Navy's presence in Sevastopol goes far beyond ships and submarines. They have a whole infrastructure, with their own doctors in their own hospitals. Even their own schools.
And that infrastructure creates employment. Local officials estimate that some 20,000 Ukrainians rely on the Black Sea Fleet for their jobs.
"If the Russian armed forces left, this town would be finished," says Natalia, a local shopkeeper.
The local economy relies on the Russian fleet
"There are so many areas like this one where everyone, all the shops, survive on the Russian military. If they left, that would be the end of us. We'd be left out on the streets, without work."
It is hardly surprising then that many in Sevastopol see Russia as a friend, and Nato as a threat.
Not far from Sevastopol is the small town of Balaclava. Like many places in the region, it has a long history of conflict between Russia and the West.
During the Crimean war, the British based their fleet in Balaclava. Then around 100 years later, the Soviets built an underground submarine bunker here. Until the end of the Cold War, it was classified as top secret.
These days, it is a museum. But, says director Oleh Kharievsky, when it comes to Nato, many people's attitudes remain unchanged.
Ukrainians face a dilemma over the future of Sevastopol
"Many people here have seen the enemy, not just through the lens of a camera, but down the barrel of a gun. So it's pretty obvious that those people's view of Nato hasn't changed in 30 or 40 years."
After the upheavals of the 1990s, Russia is reasserting itself on the international stage, and it sees the military as an important part of that resurgence.
In January, the flagship of the Black Sea Fleet was sent to lead the country's first large-scale exercises in the Atlantic for 15 years.
Russia's lease in Sevastopol runs out in 2017, and the current Ukrainian government has made it clear it is not keen to renew. The Russians, though, will not want to give up the southern port without a fight.
Ukraine may be hoping that closer integration with Nato might persuade them to change their minds.