But who is motivating him to do so

There's a mixed bag of old school neocons, bloodthirsty groypers, and profit seeking capitalists who would all love an "easy" land grab in a relatively underdeveloped region. In the same way Israelis marching into Gaza, slaughtering the residents, and sending in waves of geeked up settlers to lay claim to the turf is popular in the Middle East, you could easily find a surplus of poor, angry, desperate young men in the States to free-boot in Latin America or some desolate northern wasteland.

Would be a hellish waste of human lives and resources, but when has that ever stopped us before?

Who really needs strategic access to Greenland and the Panama canal?

Greenland has a ton of untapped mineral wealth looked under the ice sheets. Panama's strategic significance to international shipping is only behind the Suez Canal and Singapore. I'm sure Chinese shipping magnets and mining companies would love to have claims on them both. But this is more about the DC reinforcing its control of the Atlantic than Beijing making a stretch claim on the other side of the world.

china using their junior proxy ruzzia

The Soviet-Sino split isn't even cold in the ground and Model UN brainiacs think Moscow is already fully beholden to Beijing.

I'm imagining going back 20 years and asking "Why is Iran so keen on getting us out of Afghanistan? What villainy could they be planning with their dipshit lackeys in Pakistan?"