- Montenegro was formally invited to join the alliance in 2015
- Monday's event comes on the heels of Trump's controversial appearance at a NATO meeting last month
Montenegro's Prime Minister Dusko Markovic and Foreign Minister Srdjan Darmanović presented US State Department undersecretary Thomas Shannon with Montenegro's instrument of accession to the North Atlantic Treaty.
Montenegro was formally invited to join the alliance in 2015 and despite awaiting its formal acceptance, participated as a full member during the NATO leaders meeting in Brussels last month.
Monday's event comes on the heels of President Donald Trump's controversial appearance at that meeting of world leaders, where he lectured fellow heads of state over their defense spending and failed to reaffirm US commitment to Article 5 of the treaty -- a key provision that outlines the principle of collective defense.
While Trump proclaimed the trip a "great success for America," European leaders have been critical of Trump's remarks, as well as his decision to withdraw from the Paris climate accord, seeing the President's actions as signs the US is ceding its global leadership role in favor of an inward turn.
On Monday, however, acting Assistant Secretary of State for European and Eurasian Affairs John Heffern tried to strike a different tone in his remarks to the crowd of ambassadors and foreign officials, directly referencing the importance of Article 5 and collective defense.
NATO Secretary-General Jens Stoltenberg echoed Heffern's comments on the alliance's commitment to Article 5.
During last month's NATO meeting in Brussels, Trump also drew criticism after he was seen pushing aside Markovic during a photo opportunity.
Trump did not attend today's ceremony, but Markovic did visit the White House later in the afternoon and was seen leaving with Vice President Mike Pence, though neither answered questions.