As confirmed in his Masterclass The West Wing Writers' Room, Aaron Sorkin always intended for Zoe to be kidnapped not by an international terrorist organization but rather some domestic group like Rapturists just like Nancy McNally had predicted.
After Bartlet resigns the presidency, he and Leo are discussing President Walken's next move. When Bartlet says "So what's Walken going to do, cry havoc and let slip the dogs of war?", he's quoting Shakespeare's play, Julius Caesar.
This episode is the first appearance of the Bartlets' eldest daughter Elizabeth 'Liz" Bartlet Westin, played by Annabeth Gish.
Though the character of President Walken plays a prominent, key role in this episode, John Goodman remains uncredited for the role, as in the season four finale.
Aaron Sorkin has never seen a single episode from season 5, 6 or 7 of The West Wing.