This is "The Pacific Mystery" by Stephen Baxter, First published in "The Mammoth Book of Extreme SF" edited by Mike Ashley. The story also appeared in "The Year's Best Science Fiction: Twenty Fourth Annual Collection" edited by Gardner Dozois.
All this happened, you see, because the Japanese had not been able to
pose a threat to the Americans. If not for the impassibility of the
Pacific, America’s attentions might have been drawn to the west, not
the east. And without the powerful support we enjoyed from America, if
Hitler hadn’t been moved to offer such a generous peace in 1940—if
Hitler had dared attack Britain—the Germans would have found
themselves fighting on two fronts, west and east.
Could Russia have
survived an attenuated Nazi assault? Is it even conceivable that
Russia and Britain and America could have worked as allies against the
Nazis, even against the Japanese? Would the war eventually have been
It is three days since we left behind the eastern coast of Asia. Over
sea, unimpeded by resupplying or bomb-dropping, we make a steady
airspeed of 220 knots. In the last forty-eight hours alone we should
have covered twelve thousand miles.
We should already have crossed
the ocean. We should already be flying over the Americas. When I take
astronomical sightings, it is as if we have simply flown around a
perfectly behaved spherical earth from which America has been deleted.
The geometry of the sky doesn’t fit the geometry of the earth.