Barack Obama declared a “defining moment in history” on Thursday as he rode a wave of support from young and independent voters to defeat Hillary Clinton in the Iowa caucuses, putting him in pole position to win the Democratic presidential nomination.

Video: Obama wins Iowa

Lionel Barber discusses the upset in the US presidential campaign

The Illinois senator, who is bidding to become the first African-American US president, won by a decisive eight-percentage point margin, with Mrs Clinton, the senator for New York, finishing third behind former senator John Edwards.