The United States doesn't need to buy or invade Greenland to ensure security from the Chinese or Russia. Trump is lying...as usual;
"Under a little-known Cold War agreement, the United States already enjoys sweeping military access in Greenland. Right now, the United States has one base in a very remote corner of the island. But the agreement allows it to “construct, install, maintain, and operate” military bases across Greenland, “house personnel” and “control landings, takeoffs, anchorages, moorings, movements, and operation of ships, aircraft, and waterborne craft.”
https://www.nytimes.com/2026/01/07/world/europe/trump-greenland-denmark-us-defense-pact.html