He has been for a while now, which is actually a violation of the US' nato obligations. But who care anymore, the US is no longer an ally.

Edit: it also violates a ton of border treaties, and half a doze international agreements the US has benefited from for a long time. But just with NATO:

Key points of the treaty

• The parties agree to settle disputes peacefully
• The parties agree to refrain from using force in a way that goes against the United Nations' goals
• The parties agree to consider an attack on one of them as an attack on all of them
• The parties agree to consult about threats and defense matters
• The parties agree to safeguard the freedom, common heritage, and civilization of their peoples
• The parties agree to promote stability and well-being in the North Atlantic area

Threatening Canada violates all of these and then some.