Which existing treaty already gives the United States significant overflight and basing rights in Greenland, predating the September 2026 security deal?

Explanation: The United States already holds nearly total overflight and basing rights in Greenland under a 1951 treaty with Denmark, meaning the September 2026 announcement largely formalised and expanded an existing legal framework for American military presence rather than establishing an entirely new arrangement. This context is important for understanding why the new deal, despite Trump's celebratory framing, stopped well short of the outright territorial annexation he had previously threatened to pursue.