Speculative Fiction

Alternative history fiction gives a new significance to the phrase, "speculative fiction," as an alternative meaning of "sf." We can only speculate about "What if -?" And, if we imagine a world that has not yet had a scientific revolution, then we are not concerned with science - unless a scientific rationale is presented for the existence of the alternative history.

Some alternative speculations have obvious starting points. What if:

World War II had not happened?
Germany had won it?
Britain had retained its North American colonies?
The Confederacy had won the American Civil War?
The Spanish Armada had succeeded?
Muslims in Spain had succeeded in invading France?

SF writers meet the challenge of devising unobvious speculations. Harry Turtledove asks:

What if aliens had invaded Earth during World War II?

- and presents several other speculative series that I am as yet unfamiliar with.

Poul Anderson's Time Patrolmen discuss what would have happened if Cyrus the Great had not lived or if the Mongols had invaded North America and experience what happens when Carthage wins the Second Punic War.

SM Stirling asks what if:

Powerful aliens had terraformed Mars and Venus long ago?
A natural catastrophe forced the British Empire to relocate to India?
Alexander the Great had lived longer but with only a negative effect on subsequent history?
Former British Empire loyalists and mercenaries had founded an aggressively militaristic colony in southern Africa after the successful American Revolution?

That last, "Draka," history is perhaps the least obvious but has generated a four volume series.