Tillerson has written to the Senate endorsing NATO membership for Montenegro:

Secretary of State Rex Tillerson has written to the leaders of the U.S. Senate urging the ratification of Montenegro as the newest member of the NATO alliance, saying it is “strongly in the interests of the United States.”

It’s not surprising that Tillerson strongly supports NATO expansion, and there was never any realistic chance that the Trump administration would oppose Montenegro’s accession. It is still worth reviewing why Tillerson is wrong when he says that it is in our interest to bring them in. Adding a new ally will mean taking on one more security dependent that can’t pull its own weight. If the administration has any desire for current allies to share more of the burden of their own defense, backing the addition of a new member that won’t add anything to the alliance sends exactly the wrong message. NATO is already too large and unwieldy, and it has several unreliable members, and bringing in a state whose people are sharply divided over the prospect of membership will only make it more so.

Continued NATO expansion doesn’t serve American interests, unless those interests are defined so broadly as to be meaningless. Unfortunately, that is how they would have to be defined in order to believe that having another member in NATO advances them.