Recent criticism of NATO has focused on post-Cold War enlargement, questioning what new members bring to the alliance and whether it's in NATO's interest to commit to defending them.
Russia, too, has criticized this expansion. Moscow sought to halt Montenegro's ascension by fomenting a failed coup there in late 2016. North Macedonia's membership would further NATO's presence in the Balkans, an area where Russian has long held influence.
President Donald Trump — a frequent, strident critic of NATO — inflamed this debate in summer 2018, when he questioned whether the US should defend Montenegro.
Trump may have echoed a common criticism, but he was widely rebuked for seeming to undercut the collective-defense principal that undergirds NATO's existence.