A book written by Tom Clancy, and the first Jack Ryan novel to feature Ryan as a politician.
America and Japan are plunged into a war precipitated by a Japanese attack on Americas financial markets. The closing stages of this novel feature a rogue Japanese pilot plunging his jet into a full sitting of the joint houses of parliament. Ryan, though unelected, has been newly appointed to the office of Vice President when this occurs. So with the president, cabinet, and congress all dead, Ryan becomes POTUS.
This sets the stage for the sequel, Executive Orders (and later The Bear and the Dragon) where Ryan must deal with the responsibilities of being President.