2 events spring to mind.
1) July 1st 1916 (The Battle of the Somme) when my great great Grandfather, along with 20,000 other brits died at the hands of the invading Germans. I would have loved to have watched his back or perhaps just to be there to make sure his died with dignity.
2) 1932-33 England cricket tour of Oz, where we utilised the "bodyline" tactics. Loved to have supported the English in the face of great adversity from the swaggering aussies. Also would have witnessed Bradman, possibly the best batsman ever albeit an aussie.