James Potter was born c.1960 (CR) of a Wizarding family. He inherited quite a fortune (AOL). James was probably the only son in that family (OP6). While at Hogwarts (c. 1971-1978) James made quite a name for himself. He was a brilliant student who became Head Boy in his last year. He was one of the the ringleaders of a group of mischievous students and was seldom seen without his best friend, Sirius Black, by his side (PA10). One of his accomplishments, although unknown to most people at the time, was to become an Animagus, quite a feat for an underage wizard. He did this because another of his dearest companions, Remus Lupin, was a werewolf. In animal form, all of the group of friends could join Lupin in adventures around the countryside once a month when he had transformed.
James was an excellent Quidditch player. He was a Chaser for the Gryffindor team (Sch2). He received an award for Quidditch in 1973 (SS/f).
(NOTE: In the film, James is said to be a Seeker (SS/f), but JKR stated that he was the Chaser in the second Scholastic interview).
At Hogwarts, James met Lily Evans, a Witch of Muggle parentage, who was also quite a brilliant student. At first she would have nothing to do with James, who was somewhat arrogant (OP28). Eventually the two became a couple and by the time they were seventh years, they were Head Boy and Girl. Lily and James were married straight out of Hogwarts (c. 1979) and had a son, Harry, on July 31, 1980. James and Lily were strongly loyal to Dumbledore in the fight against Voldemort. They were members of the Order of the Phoenix and fought against the Dark Lord (OP). When Voldemort became aware of the prophesy concerning a baby born at the end of July of 1980, they became a particular target of the Dark Lord, and they went into hiding in 1981.
SO as far as i can tell they didn't actually defy him, it was the prophecy that sparked voldemorts targeting of the potters