"The hashtag surges with each new case of a police killing of a black person that is seen as unjust, usually after a video emerges. This week it surged twice - with the fatal shootings of Philando Castile in Minnesota and Alton Sterling in Louisiana. Each time it surges, mostly peaceful protesters come out in US cities to declare that "Black Lives Matter"...

It's such a powerful rallying cry that some have controversially likened it to a reborn US civil rights movement. That's what was happening in Dallas on Thursday night - a peaceful Black Lives Matter protest - until suddenly, it was anything but.

The sniper fire at that protest eventually led to the murder of five Dallas police officers and the wounding of seven more. Two civilians at the protest were also wounded. And now, the phrase Black Lives Matter is suddenly on everyone's lips - and not in the way its supporters would like."

Read more from BBC Trending on the meaning of Black Lives Matter.