BRUSSELS – The small European country now known as the Republic of Northern Macedonia cannot act as a major addition to NATO. But it joins another hole in what was once the former Soviet Union's backyard, and comes at a time when the east-west tensions rise.
Involvement of Northern Macedonia also means that NATO will soon have 30 members, all engaged in their collective defense. That is twice what NATO's membership was in 1991 when many experts believed that the alliance would break down with the Soviet Union and the Warsaw Pact.
At a ceremony at NATO headquarters in Brussels, the member states formally signed a letter of accession for Northern Macedonia, which was to become a full member at the end of the year.
But with Russia working openly to restore its influence in the former Soviet sphere, most lively in 2014 with the annexation of the Crimea and the ongoing war in eastern Ukraine, NATO is an attractive club for small nations like Northern Macedonia.
"One of Putin's greatest hopes has been that NATO would go away or be fragmented," said Stephen J. Flanagan, senior political scientist at RAND Corporation in Washington. The accession of Northern Macedonia, he said, "presumably undermines the Russian narrative."
More troubling for some NATO supporters may be President Trump, who has complained that other member states are not spending enough on defense. He has referred to the alliance as "obsolete" and even threatened to retire.
Mr. Trump also clearly worries that small NATO countries in difficult neighborhoods could drag the United States to war under the Alliance's collective security clause known as Article V: An attack on one is an attack against everyone.
In July, Mr Trump proposed that NATO's 29th member, Montenegro, a 600,000-person country, which he describes as "very aggressive", could start the Second World War.
Herr. Trump's views questioned whether he was obliged to come to the aid of another NATO Member State if it was attacked.
Torrey Taussig, a fellow at the Brookings Institution based in Berlin, said that although the Trump administration has a lot of diplomatic support behind the Macedonian road to NATO, there is doubt about Mr. Trump's commitment to Article V has not gone away.
"When the central deterrent node is called into question, any NATO state security is assured into the issue with it – whether NATO should have 10, 30 or 50 members, she said.
Kay Bailey Hutchison, the US Ambassador for NATO, has said that the United States supports "strong" membership of Northern Macedonia and its 2.1 million people.
In addition, Congress and Pentagon remain committed to an alliance that supporters have long credited to help keep the peace of Europe and discouraging Moscow for the last seven decades Dec.
Only last month, the House of Representatives adopted a two-party measure with a vote of 357-22 to ban the Trump administration from using federal funds to withdraw the US from NATO and dispute the president of his frequent attacks against the alliance and to suggest that he may try to retire.
Russia's efforts to see disagreement in the alliance have appeared on Balk where it has promoted its influence in the former Yugoslavia, of which both of the newest NATO members were part. Russia was accused of trying to overthrow the Montenegrin government in October 2016, for example, in a last-minute effort to stop the country from joining NATO.
In August, Greece accused worrying Russia of interfering before an important vote in the Greek Parliament on a change of name for Macedonia, which will pave the way for its accession to NATO. Greece, which has historically had a quiet relationship with Russia because of their Eastern Orthodox constituencies, showed four Russian diplomats.
Despite these setbacks to Russia, it continues to exert significant influence in Serbia, where Mr. Putin has just made state visits and where Russia has great interests in infrastructure. The persistent conflict between Serbia and its former province of Kosovo, which Serbia and Russia refuse to recognize as an independent state, has kept Kosovo from seeking to join NATO.
Nikola Dimitrov, Northern Macedonia's Foreign Minister, said that NATO membership would place land in "a stability zone in a region that still has uncertainty pockets".
Membership was so valuable to the country, as it adopted after many years of negotiations with Greece, a NATO member, to change its name from the Republic of Macedonia. The Greeks claimed that they already had a northern territory called Macedonia, that historically Alexander was the Great Greek – Alexander of Macedon – and that the ancient kingdom of Macedonia sits in modern Greece.
Until the name change, Greece had vetoed Macedonian membership in NATO, much to the annoyance of the rest of the alliance.
For years, the name of the question had kept the Macedonians from trying to become a member of the European Union, as it did by consensus, like NATO. Only when Greece has lifted its veto can actual accession negotiations with the EU begin.
"This is really a big moment," said Stevo Pendarovski, a Macedonian official who coordinates the country's accession efforts for NATO.  While NATO is proud of its expansion, NATO members remain cautious with Mr. Trump's anger and his habit of interrupting the procedure for divesting his allies deficiencies, which was shown at the last NATO summit in July.
This April is the 70th anniversary of the alliance, which would normally be celebrated at a summit of all NATO leaders in Washington. This year there will be no such gathering in Washington, but a meeting instead of foreign ministers.