ISTANBUL — After a career at sea and eight years of retirement, Cem Gurdeniz, a 62-year-old Turkish admiral, has suddenly found himself in the limelight here, touting an expansive, nationalist vision of Turkish power projected far into the contested waters off his country's shores.

Gurdeniz developed the maritime doctrine, called Blue Homeland, more than a decade ago because he was disturbed by what he said was the government’s reluctance to secure Turkey’s rights. His vision has gained popularity at a volatile moment as Turkey and Greece square off in the eastern Mediterranean, leading to fears of a war within NATO.

Blue Homeland’s aims are spelled out on a map showing Turkey’s land mass surrounded by a wide buffer of nearly 180,000 squares miles of sea stretching beyond the Greek islands off Turkey’s west coast. The concept — once narrowly associated with left-wing nationalists — is now regularly cited by Turkish officials, including President Recep Tayyip Erdogan, when talking about maritime disputes. Blue Homeland has energized Turks who feel the country has been unjustly denied its rightful claims to the sea, given its long coastline, and has confirmed for adversaries fears of resurgent Turkish expansionism.

“We cannot neglect the seas again. We cannot be pushed away from the geopolitics of the Mediterranean, the civilization of the Mediterranean,” Gurdeniz said in an interview in an Istanbul cafe overlooking the Bosporus and, in the distance, the Black Sea.

Rival claims by Turkey and Greece over sovereignty in the island-dotted seas that separate them have set off a squall, marked by taunts, denunciations, rival maps and aggressive deployment of warplanes and ships.

In the last few months, tensions have centered on the Oruc Reis, a Turkish seismic research vessel that has been exploring for oil and gas deposits in contested waters while escorted by Turkish naval ships and stalked by Greek frigates. Greek and Turkish naval ships collided in mid-August, heightening concerns of a wider conflagration.

The conflict has cleaved the region into feuding camps, pitting Turkey and Libya against an alliance led by Greece, Cyprus, the United Arab Emirates and Egypt. France has sided with Greece, and the United States finds itself stuck between its NATO allies.

Increasingly bellicose attitudes toward Europe among elements of Turkey’s political establishment have proved fertile ground for promoters of Blue Homeland. “Just as our nation achieved victory in its fight for independence despite poverty and deprivation, it will never hesitate to thwart the desires and moves for a Sèvres in Blue Homeland today as well,” Erdogan said in a speech last month, referring to the Treaty of Sèvres, which divided up the Ottoman Empire among European powers.

Turkey’s Defense Ministry has referred to Gurdeniz’s vision — Mavi Vatan in Turkish — as its “covenant.” The admiral has become a frequent guest on television talk shows. Blue Homeland has seeped into the culture as well, featuring, for instance, in a recent radio commercial for a Turkish solar panel company.

There is “significant evidence that suggests that Gurdeniz’s views have had a profound impact,” Ryan Gingeras, a professor at the Naval Postgraduate School in Monterey, Calif., wrote in a June essay about Blue Homeland that noted its widespread use in the Turkish political establishment and among other former senior naval officers.

A clear sign of the doctrine’s influence was a maritime agreement Turkey struck with one of Libya’s two warring governments last year that seeks to extend Turkish jurisdiction far into the Mediterranean, south of Crete.

Kyriakos Mitsotakis, Greece’s prime minister, writing this month in several European newspapers, called Turkey’s agreement with Libya “illegal” and cited a litany of provocative actions carried out by Erdogan, including hydrocarbon exploration in disputed waters. “Turkey’s rhetoric is from a bygone age,” Mitsotakis wrote. “It talks about enemies, martyrs, struggle, and a willingness to pay any price.”

A recent announcement by the Trump administration that it would conduct military training with Cyprus, Greece’s ally, angered Ankara. In the last few days, the United States has taken the unusual step of denouncing a rival map that has been used to justify claims by Greece and Cyprus to broad swaths of the sea — a move intended to assuage Turkish fears.

“The United States does not regard this document as having legal significance,” David M. Satterfield, the U.S. ambassador to Turkey, said during a meeting with journalists Tuesday, referring to the Seville Map commissioned more than a decade ago by the European Union.

“This cannot be resolved by declarations, nor can it be resolved by production of maps or other documents,” he said.

On Tuesday, in what seemed like a breakthrough, Turkey and Greece agreed to start a new round of negotiations “in the near future” over their contested maritime claims, Greece’s Foreign Ministry said in a statement.

But in advance of any talks, Turkey and Greece have staked out “maximalist positions,” according to Sinem Adar, an associate at the Center for Applied Turkey Studies in Berlin. “Reaching a compromise will be a hard and long path if it ever happens,” she said.

Arguments over maritime claims have brought the two countries close to blows before, including in 1996, when the United States stepped in to defuse a conflict over a 10-acre uninhabited island. The stakes are higher now, because of the scramble for oil and gas deposits in the contested waters around Cyprus.

The disputes will be solved only when Ankara and Athens show a willingness to compromise, analysts say, but so far, the two governments have been unable to even agree on the ground rules, with Turkey rejecting definitions laid out in the U.N. Convention on the Law of the Sea, which it has never ratified, and Greece saying its claims — including to expansive jurisdiction around its many islands — apply to Turkey as a matter of settled international law.

Before Blue Homeland was widely adopted by Turkish politicians, it was seen as reflecting the worldview of nationalists who oppose Ankara’s orientation toward NATO, the United States and the E.U. and favor closer ties with Russia and China. Gurdeniz — who studied at the Naval Postgraduate School, worked at NATO and collaborated extensively with U.S. naval officers — said he agreed with those views but is not a “zealous” nationalist. He called himself a “Kemalist,” referring to the secular ideology of Mustafa Kemal Ataturk, the founder of modern Turkey, and distancing himself from Erdogan’s Islamist-based policies. But he added: “I don’t get involved in the daily politics of Turkey.”

Gurdeniz joined the Turkish navy in 1972 as a 14-year-old cadet, became an officer seven years later and was promoted to admiral in 2004. He came up with the concept of Blue Homeland while working in the navy’s policy and planning office in 2006, drawing inspiration for the phrase from his late mother, who was a “maritime poet,” he said.

In the interview, Gurdeniz laid out his doctrine’s lineage, a history of grievances stretching back to the Ottoman era that he said showed how Turkey had missed opportunities to exert its maritime claims or been unfairly hemmed in by foreign powers. A turning point, he said, was Turkey’s 1974 invasion of Cyprus and the division of that country between the Greek Cypriot south and the Turkish Cypriot north. Only Turkey recognizes the northern government. “Turkey changed the map,” he said, referring to the invasion as “one of the biggest achievements in military history.”

In 2011, Gurdeniz was among hundreds who were arrested in a purge of nationalist officers and convicted of plotting to overthrow Erdogan’s Justice and Development Party. He said he promoted his ideas about naval power at his trial and later, as he served 3½ years in prison, in a column he wrote every Sunday for a newspaper affiliated with Vatan, a Turkish nationalist party.

But it was not until after a coup attempt against Erdogan’s government in 2016 that the concept of Blue Homeland really took off, Adar said, as Erdogan formed a political alliance with nationalists and embarked on a more aggressive foreign policy that has seen Turkish armed forces engaged in conflicts from northern Iraq to Libya.

She said Turkey’s ruling circles had concluded after the coup attempt that “Turkey is under threat. The global order is changing. We can’t trust our Western partners. We have to help ourselves.”

It is unclear, however, how long Blue Homeland will remain popular. A recent poll by the Turkish research group Metropoll showed the Turkish public overwhelmingly opposed to a military conflict in the eastern Mediterranean, Adar said. Among Turkey’s ruling elites, a central disagreement has been whether to emphasize diplomacy with Greece or continue to lean on military power. Another point of divergence is Turkey’s relationship with Egypt, with some — including Gurdeniz — arguing Turkey should mend fences with Egypt’s military-backed government, which is an adversary in the current crisis but some view as a natural ally.

Ultimately, the question for Gurdeniz is how to resist adversaries that want to see Turkey “landlocked.”

At stake, he said, is Turkey’s defense, its security, its access to resources and its welfare. “Even happiness,” he said.