A quick outline of the Stross series:
[spoiler]There are an extended family of people in a parallel world that is at a medieval level of development who have the inborn ability to jump between their world and ours when they concentrate on a certain pattern. The ability has limitations–you can only carry as much as you can–uh, carry from one world to the next, and making the jump causes serious headaches and can eventually be fatal as you get older. But the family uses the ability to import goods from our world into theirs, and to take advantage of the modern world’s transportation system (take a plane from coast to coast instead of a horse.)
The US government on our side eventually discovers these people, and discovers that the ability to cross between worlds comes from nanotech engineered into their mitochondria by people in a timeline much more advanced than ours. They are able to culture cells on a chip to trigger their own ability to travel between worlds on our own using gadgets.
So late in the first series of novels, an ambitious stepper from a radical faction on the medieval world takes a stolen satchel nuke (stepping between worlds lets you get into nuclear storage facilities) as blackmail in our world. But the US government isn’t very happy with the blackmail–they find the satchel nuke and give it back to the steppers through one of their known access points–with the timer ticking down. Destroys a castle, kills a bunch of people. The faction decides to punch back–they bring another nuke into DC, set it off in the parking garage of the Pentagon, destroy the government area of DC and kill Bush 43. The faction thought like medieval people–kill one king, the next one will have more respect for you. New President Chaney didn’t think like medieval, but did think about going medieval on their assess–a fleet of nuclear bombers with stepping gadgets carpet bomb the east coast of North America in the medieval world. The extended family of steppers escape to a recently-discovered 3rd world with a WW1-ish level of tech but a 17th century culture.
A new series is currently in progress centering around the discovery of artifacts from one of the vastly more advanced but abandoned timelines.[/spoiler]