The poem is based on an actual historical event: the Charge of the Light Brigade during the Battle of Balaclava (which happened during the Crimean War). This went down in Crimea in 1854.
Here's the quick and dirty version of the history: The Crimean War, which took place between 1853 and 1856. This was essentially a battle between Britain (with its allies) and Russia for control over the territory occupied by the crumbling Ottoman Empire. In late 1854, the allied troops tried to capture the Russian city of Sebastopol, and the Battle of Balaclava was one of several fights in that campaign. During this battle, the British commanders ordered a disastrous charge by the Light Brigade, which took many casualties. That's the basic story. For history buffs, the web is packed with great resources about the Crimean War (we've linked to a few in our "Best of the Web" section).
The most interesting part of the setting, though, is definitely that "valley of Death" – check out "Symbolism, Imagery, Wordplay" for some discussion on that.