Military History
The National Archive at Kew holds a lot of records, as does the Army Museum. Ancestry has a large collection of World War 1 documents which of course are searchable using a computer. However about a third of all WW1 records were lost making it a matter of chance whether the ones you need have survived.
With ancestors who served in the Army, local regimental museums hold large collections of records. In the 1870s, Cardwell introduced a number of long overdue reforms to the way in which the Army was structured. Cardwell’s Army Reforms included a major restructuring of individual regiments.
Everyone of us will have ancestors who have served in the military and there is a wealth of information that can be researched into where they served and where.