To quote an old soc.history.what-if post of mine:
"In 1920 the Danish government asked the UK to recognize its right to extend
its political and economic interest in the whole of Greenland--a claim to
sovereignty already acknowledged by the US as a condition of the cession of
the Danish West Indies four years earlier. The British government replied
that it would agree to this proposition only if granted the right of pre-
emptive purchase in case Denmark should consider disposing of Greenland.
When word of the British demand reached Washington, Secretary of State Colby
strongly objected, and in deference to the US objection, the UK softened its
conditions.
"Even in 1940, when one might think after Hitler's occupation of Denmark, the
US might welcome a British or Canadian occupation of Greenland, instead the
US was anxious to prevent precisely this event, while not yet ready to
dispatch troops itself. (Eventually it did, of course, but only after
keeping the question in suspense for a year.) This was partly out of a
desire to deny Japan an excuse for a 'protective' occupation of the Dutch
East Indies should Hitler make his expected assault on Holland. But it was
also a product of the US belief that Greenland was part of the Western
Hemisphere, and that the Monroe Doctrine (including the no-transfer policy)
applied. Hull specifically reminded Lord Lothian of Colby's 1920 note, which
Hull called an 'express application of the Monroe Doctrine by the United
States.'"
https://groups.google.com/d/msg/soc.history.what-if/Y6b49i2xeeI/Kr2WwQ-NuTIJ