Josh Marshall is perfectly right to make this argument that there is nothing like a NATO fund to which anybody could owe back payments.
But Trump is saying that over all that time that country X did not carry its weight everybody else, mostly the US, was carrying its weight.
And for that payback is due, not to some imaginary NATO defense fund but to the US.
He just hasn't made the point completely clear.
In his view, each member of the NATO alliance should spend as much on its own defense as we spend on its defense - not absolutely in dollars, d-marks, or whatever (so far, anyway), but according to its ability, as a percentage of GDP.
And if they haven't they owe us.
Not an easy point to dispute.
Which is not to say it's indisputable, but so far the left is avoiding even engaging the point by instead leaning heavily on the always entirely believable "Trump is ignorant, confused, and incoherent" line.
Worse than John McCain in a senior moment.