update: Jan. 3, 2019
Update 4/12/2017: Pres. Trump agreed to admit Montenegro into NATO, expanding NATO.
There is a Russian/Manafort connection with Montenegro dating years ago, but before NATO is an application for membership. At their first NATO meeting during the time of the Trump administration,
Secretary of State Rex Tillerson will not attend and will be in Moscow instead. The Russian interest in Montenegro is historic. Sen. McCain has been interested and knowledgeable about Montenegro and got embroiled over a spat with Libertarian isolationist Rand Paul. The spat got personal. The following post is from a facebook posting recently. The Manafort Russian connection is most likely the subject of the FBI investigation into Russian influence in the US Presidential Elections in 2016.
Per Balkan Reports, the impression of those in the Balkans is that Secretary of State Tillerson supports Montenegro's membership in NATO. The entire Balkans are concerned with Russia's increasing influence and presence in their region and look at this as a way to counter this.
The Manafort/Russian/McCain issue connection was described in https://www.thenation.com/article/mccains-kremlin-ties/