NATO's mission was to secure peace in Europe, promote cooperation among its members and guard their freedom by countering the threat posed by the Soviet Union at the time. This latter point was critical—by signing onto NATO, the United States agreed to accept the leadership of a burgeoning resistance to ostensible Soviet aggression and subversion transpiring in Europe in the aftermath of World War II. But since the collapse of the Soviet Union at the end of 1991, NATO has been an organization in search of a mission.
NATO's original parameters still apply, but for all the wrong reasons. "Keeping the Russians out" is more an economic argument than a political one, especially when it comes to Russian energy (one need only witness the ongoing angst within the EU and NATO over the Nord Stream 2 gas pipeline project), while "keeping the Americans in" has similarly devolved into an economic argument surrounding the high financial cost of sustaining NATO, and the American perception that its European partners are not paying their fair share of the bill.