Warning: contains spoilers for Avengers #57!
Marvel has officially swapped Captain America with Doctor Strange in World War II, and the end result is one of the oddest developments in the Marvel Universe in years. The company often experiments with these so-called What If? scenarios (such as the much-maligned issue in which Miles Morales became Thor). But in Avengers #57, there is no What If? scenario - only a parallel reality in which the Soldier Supreme fights for the United States of America against demonic Nazis - controlled by the demon Mephisto.
Captain America is a frequent protagonist of What If? stories. There are scenarios in which Rogers wasn't frozen at the end of the war and lived through the 50s and 60s, or a universe when he fought in the Civil War instead of World War II, and even stories in which a different person became the patriotic Avenger altogether (Miles Morales becoming Captain America is only the most recent version). Marvel has even put Captain America and Doctor Strange together as one character before - though perhaps not like this.
In Avengers #57, written by Jason Aaron with art by Javier Garron, the protector of the Allies is Sergeant Szardos, a magical sergeant in the United States Army who fights demonic Nazi machines, like a possessed submarine. Armed with a living gun, Szardos' World War II is quite different from the conflict known by modern-day audiences. He battles possessed soldiers, hellish birds, and even has a dogfight in the sky with Mephisto, performing a mid-air exorcism with the Eye of Agamotto.
At the end of the story, Mephisto is vanquished (by the combined power of the Vishanti and the United States military, no less) and Sergeant Szardos is reunited with the Invaders of his realm: a tank-driving Ghost Rider, War-Thing (a Man-Thing variant) and Blade. The character is far less composed than either Rogers or Strange, and actually has elements of Sergeant Rock of DC Comics fame. His communications with the Ancient One suggest he is new to the mystic arts, and isn't a lifelong practitioner like Baron Mordo or Clea.
The current run of Avengers comics involves other universal variants of preexisting characters fight variants of Mephisto from across the Multiverse that take on new, horrific forms. For Jane Foster in another universe, Mephisto traps her in a domestic housewife hell. For this new version of Captain America and Doctor Strange, Mephisto has elected to imprison Sergeant Szardos in a neverending war - one that even his magical abilities cannot even hope to conclude peacefully.