From Mr Kaplan’s article:

“There is nothing in the treaty that places any limits on the U.S. missile-defense program. “

Yet there seems to be a difference in how the Russians see that.

The treaty, “may be effective and viable only in conditions where there is no qualitative or quantitative build-up in the missile defense system capabilities of the United States of America. Consequently, the extraordinary events referred to in Article XIV of the Treaty also include a build-up in the missile defense system capabilities of the United States of
America such that it would give rise to a threat to the strategic nuclear force potential of the Russian Federation.”
docs.google.com