This was a story I read a long time ago, I believe from Asimov's magazine in the early 1980s (1982?), about a pilot or soldier in an alternate World War I that lasted much longer than it did in our world (at least 20 years).
The main character finds a way to possibly end the war by sending himself to an alternate reality. He does this by sending up a flare that would signal the end of the latest battle, and ends up in a world where World War II lasted longer, into the late 1940s.
There, he has an encounter with a black American pilot flying a WW2 era aircraft which the protagonist notes is more advanced than those he is familiar with.