WASHINGTON – NATO on Wednesday formally invited Macedonia to begin the process of becoming the 30th member of the alliance – but with a strict warning that internal politics could scuttle any agreement.

“NATO’s door is, and will remain, open,” NATO Secretary General Jens Stoltenberg announced following the first day of the Brussels Summit. “Today we agreed to invite the government of Skopje to start ascension talks.”

“Once all national procedures have been completed to finalize the name agreement, the country will join NATO as our 30th member,” he added.

That may be easier said than done, however. Greece has held up Macedonia’s requests to join NATO and the EU for decades, as part of a demand that its northern neighbor change or modify its name to avoid any claim to the territory and ancient heritage of the region in northern Greece named Macedonia — birthplace of ancient warrior king Alexander the Great.

Under a new agreement made last month, Greece’s Alexis Tsipras and Macedonian Prime Minister Zoran Zaev said the former Yugoslav republic’s new name for both domestic and international purposes would be Republic of North Macedonia. Macedonia will also amend its constitution to reflect the change as part of the deal.

However, Macedonia must go through a voter referendum, and the issue over the name change is politically charged. The referendum vote is expected in late September or early October.

Speaking on the sidelines of the event, Zaev made it clear he was thrilled with today’s formal declaration, and threw his full support behind NATO, calling it a force for world stability that “we want to participate” in.

Zaev also expressed optimism that the voters will back his plan, despite opposition from Macedonia’s President and other political figures.

“After the voting of our citizens in a huge majority, the politicians must obey completely and fulfill the intention of the citizens,” he said.

Supporters of the name change certainly hope to see an easy win in the referendum, because this may be the one shot they have at joining the alliance.

During a press conference, Stoltenberg was asked about the referendum and grew visibly agitated, banging on the lectern and raising his voice in a way unusual for the steady NATO head.

“This is a once in a lifetime agreement. Either they support the agreement and then they can join NATO, or they don’t support the agreement, but then they will not join NATO. They cannot get both. There is no way to join NATO without changing the name,” Stoltenberg said.

“It can’t become a member if it does not change its name. That is a simple choice. And it’s up to the people of the Former Yugoslav Republic of Macedonia to make that choice.”