If we don’t want to defend them, don’t let them into NATO. If we are going to let them in, then yeah, we have to defend them, otherwise the alliance is pointless.
The country was invited to join the Membership Action Plan in December 2009.
Montenegro was invited to start accession talks to join the Alliance in December 2015 and the Accession Protocol was signed by Allied foreign ministers on 19 May 2016.
The Accession Protocol then had to be ratified by all parliaments in NATO member states.
Montenegro became a full member of the Alliance, when the instrument of accession was deposited in Washington D.C. on 5 June 2017.
It appears that by the time President Trump took office, the 0bama regime had already done their dirty work, though congress could've stopped it. My guess is that several members of the congressional "leadership" were royally wined and dined and approval was merely a formality.