Canada

Canada has a ton of untapped natural resources that he wants to irresponsibly harvest and sell for the fastest profit and growth. A lot of these resoucres are hard to access due to how northern they are but melting sea ice is making northern ports and shipping routes more feasible, increasing access to these remote resources. The times he refers to as "america being great" were times of heavy natural resource extractions and little to no environmental regulations. He is also eying Canada's vast fresh water supplies.

The Colorado river has customers purchasing more water than the river even has flowing in it. On top of this, very poor water regulation policy means farmers are encouraged to use as much water as they can rather than preserve water when possible because unused water may be taken off of next years allotment. Instead of reworking these policies and discouraging water intensive agriculture cash crops in the area, he'd rather just take Canada's water.

A big part of this may have started from his ego but strategically America absolutely stands to benefit economically, militarily, and food security wise from annexing Canada. Canadians cannot afford to take these threats lightly.