Bernard Cornwell says that British soldiers lived in their regiments during the Napoleonic wars, and it seems brigade and division assignments would change each engagement. The same seems to be true (in fiction) about the US Civil War. However, in WW2 stories the soldiers, at least the army, seem to identify with their divisions, not regiment.

Now days it seems military personnel transfer every few years (that's based on knowing some senior non-coms hanging on for their twenty years (and I spent part of my childhood in West San Antonio, so my impression passes the twenty year rule, though only for the air force)).

So, was the regiment that much more important to the soldier in the nineteenth century, and how and why did it become so unimportant by WW2 and now?