What's a bone of contention?
Contention is a disagreement and I'm sure you know what a bone is.
Is it to do with dogs fighting over a bone?
It's something that has been a long term cause of dissatisfaction and is an unresolved conflict.
One such contentious issue is the Falkland Islands or Malvinas, as the Argentinians call them. Lying off the coast of Argentina, they were colonised in 1833 but forcibly reclaimed by Argentina in 1982. This sparked a war. My country at war in my lifetime - a strange feeling.
It's hard to find any videos that aren't very biased one way or the other but this isn't too bad. The sound track is a bit loud, so turn your volume down. I hope you find it interesting.