He thinks speaking diminutively to people automatically puts him in a position of power over them

It's an insecurity issue common with bullying

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.

Both, he honestly believes that he will take Canada and Trudeau will be demoted to a governor somehow.

Trudeau should call him Mayor, or even better, "Convicted Rapist and Felon".

Canada at least has a "Governor-General" who is appointed by the British monarch.

Edit: like several other commonwealth states, Canada is a completely separate country from the United Kingdom. It just happens, by law, that King Charles III is the king of all of them.

Carpet bomb them with inaccuracies and they'll spend too much time correcting you to keep up.

It's the new "we'll build a wall, and mexico will pay for it" - a clause so dumb that people will complain about that, unknowingly tugging "normalcy" a step in that direction.

And Trudeau called him "a special American, [...] Donald" :D