Some excellent resources and information is available on the National Amy Museum's website
http://www.national-army-museum.ac.uk/
Some useful links are...
Crimea
Florence Nightingale
Early photography (including Fenton)
Boer War
The Victoria Cross
WW1
Tanks -
Western Front 1918
Finding the Fallen
Specific teacher links for visits and resources - http://www.national-army-museum.ac.uk/education/secondary13.shtml
