Macedonia and Bulgaria's long-simmering dispute over the Macedonian language is threatening to boil over again, just as Skopje nears a resolution to its dispute with Greece over the name "Macedonia."

Greece and Macedonia's name-change deal in June, the so-called Prespa agreement, aims to alleviate Athens' concerns about potential territorial claims from Skopje -- clearing the way for the former Yugoslav republic to move forward with its efforts to join NATO and the European Union.

But a junior partner in Bulgaria's coalition government, the Bulgarian Nationalist Movement (VMRO-BND), raised the possibility this week of new hurdles for Skopje by threatening to withdraw Sofia's support for Macedonia's Euro-Atlantic integration.

The VMRO-BND, led by Defense Minister Krasimir Karakachanov, was irked over recent arguments made by Macedonian Prime Minister Zoran Zaev about the existence of the Macedonian language.

Skopje insists Macedonia is a distinct South Slavic language that forms part of country's culture and national identity.

Bulgaria was the first country to recognize Macedonia's independence from Yugoslavia in 1992, but it has never recognized Macedonian as a unique language.

Sofia insists the official language of Macedonia is merely a regional dialect of Bulgarian.

Zaev recently raised the language issue while campaigning for a constitutional amendment needed under the deal with Greece to change country's name to the Republic of North Macedonia.

In early December, Zaev told parliament the deal included clarification on the existence of a separate Macedonian language -- a reassurance that is important to the national identity of Macedonians. Zaev's remark referred to part of the Prespa deal under which Greece agreed to recognize the existence of the Macedonian language at the United Nations.

The Macedonian parliament on December 3 approved a draft constitutional amendment on the name change. A final vote on whether to adopt the constitutional amendment is expected in January.

But Zaev's remark to parliament about the language was also heard in Bulgaria, leading to outrage from Karakachanov and other Bulgarians who view events during the Byzantine era in what is now Macedonia as an important chapter of Bulgaria's early history.

Karakachanov told Bulgarian National Television last week that Macedonia was playing "tricks" and "falsifying history" to force the idea of "a Macedonian identity and language not only within Macedonia, but also on Bulgarian territory."

"North Macedonia is a geographic denomination that also includes current territories of Bulgaria," Karakachanov said. "Mr. Zaev could as well ask that the Bulgarian dialect they use be studied as an official foreign language in Bulgaria. This is unacceptable. This is a provocation."

The VMRO-BND joined Bulgaria's coalition government with pro-Western Prime Minister Boyko Borisov despite the VMRO-BND's opposition to EU and NATO expansion.

However, some politicians from Borisov's pro-Western GERB party have joined Karakachanov to protest Zaev's remarks on the language issue.

They include Andrey Kovatchev, a Bulgarian member of the European Parliament who described Macedonia's language as "the Western Bulgarian dialect," and accused Zaev of "scoffing about our ancestors' memory."

Macedonia's Foreign Ministry was quick to jump into the fray to urge calm.

It warned that a "similar counter-response" by Macedonian officials would create a "chain of negative reactions that will separate" Macedonia and Bulgaria, creating "hostility instead of friendship."

'Friendship' Agreement

Macedonia's Foreign Ministry also pointed out that a friendship treaty signed by Skopje and Sofia in August 2017 was meant to end years of diplomatic tensions.

"We have signed a friendship agreement and the friendship that we build will encompass mutual understanding, respect, and care for the neighbor's interests," the ministry said in a December 10 statement.

The bilateral agreement was praised by the EU as a diplomatic breakthrough.

Neither side has backed down from their entrenched positions on the language dispute, however, with negotiators sidestepping the issue.

The growth of deeper diplomatic and economic ties between the Balkan neighbors had been stunted by the dispute until an agreement was reached in February 1999 on how to skirt the issue.

Until then, Sofia had feared that recognition of Macedonian as a unique language would set a precedent allowing Skopje to make future territorial claims in southwestern Bulgaria -- where a similar dialect is used.

Skopje also refused to recognize Sofia's position out of fears of potential Bulgarian claims on its territory.

Critically, both sides agreed to sign future treaties "in the official languages of the two countries."

Accordingly, the carefully worded 2017 friendship treaty states: "The agreement is signed in two original copies, each in the official languages of both sides -- the Macedonian language according to the Constitution of the Republic of Macedonia and the Bulgarian language according to the Constitution of the Republic of Bulgaria."

Common History, Differing Viewpoints

The language issue isn't the only contentious issue in Bulgarian-Macedonian relations. Disputes have also arisen over differing points of view on historic events in the Balkans.

One key debate focuses on Bulgarian military forces deployed in what is now Macedonia during World War II, when the Bulgarian monarchy was allied with Nazi Germany and the Axis powers.

Macedonian schools since the end of World War II have taught that the Bulgarian troops were "fascist occupiers," while Bulgarian schools teach that they were "liberators."

Another historical dispute involves Ottoman-era revolutionary movements that both countries now claim as part of their own history.

In fact, Karakachanov's nationalist VMRO-BND claims it is the successor of the historic Internal Macedonian Revolutionary Organization (VMRO) -- a group founded in the 1890s that initially aimed to gain autonomy from Ottoman rule in the regions of Macedonia and Adrianople.

Under the friendship agreement, a Bulgarian-Macedonian Historical Committee has been set up with experts from both countries, who are trying to align their history books as much as possible to reduce future tensions.

Following two rounds of talks behind closed doors, interviews by RFE/RL with committee members suggest their discussions on some historical events encountered some sticking points.

"Macedonian textbooks will not present historical events according to the Bulgarian template," Macedonian committee member Petar Todorov tells RFE/RL. "But the new textbooks -- we will try to overcome the disputes and have them written in a way that helps build a long-term and lasting peace."

"We'd like the new textbooks to develop critical thinking by Macedonian students," Todorov adds. "That's something that we cannot find at the moment in Macedonia or in the region."

Todorov accepts that Bulgaria and Macedonia share a common history. But he insists that Macedonians also have their own language and national identity that is independent of historical ties with Bulgaria.

"A common history doesn't mean 'one people, two states'," Todorov says. "There is a common history between Bulgaria and Macedonia, but it is the history of being under Ottoman rule."

"We can also say that Macedonia and Serbia had a common history during the time of Yugoslavia."

Written and reported by Ron Synovitz, with additional reporting by RFE/RL's Balkan Service