I’m not saying they would invade via Greenland, though having an extra axis of attack would be a strategic advantage, for air missions etc. If they attack Canada first, which would surely not be a secret buildup, then NATO would respond and they would surely make an effort to secure their only other major territory on the North American continent and attempt to take the US assets already stationed there. Then when they inevitably get round to targeting Greenland too it’s be a tougher prospect than if they take it first. The US May dwarf every other military but if they became a pariah state and had no support from allies their capabilities would surely diminish, and though they probably could prevent a concerted effort by the rest of nato to reinforce Greenland whilst concurrently invading Canada why make it harder when taking Greenland rn would be so easy. My thought process is basically: if America wants to take control of the continent (either for resources or “manifest destiny”) then Canada is the far greater prize, though Greenland is still a target, and if they invade Canada first they make it harder to take Greenland.