Forty years ago National Guard troops in the US state of Ohio fired on students who were protesting against the Vietnam war at Kent State university.<br /><br />Four students were killed that day, piercing the conscience of people across the country.<br /><br />It became a defining moment for the US anti-war movement.<br /><br />Al Jazeeras Nick Spicer looks at what the events of Kent State mean for today (4 May 2010).
