Our military and maritime history pages have a wealth of links to resources which describe wars throughout history and information on how to obtain records of service.
FTF Guide: Military Research A detailed overview of where to look for records.
FTF Guide: Maritime History The central page for maritime information.
FTF Guide: Military History The central page for military information.
The Royal Navy, The British Army and the The Royal Air Force each have their own dedicated page. There are separate pages for World War One and World War Two.
Resources for the Allied Forces can be found on the page for each nation:
In addition, there is a page dedicated to those who lost their lives in the service of their country on War Memorials
You may also find these editions of the Family Tree Forum Online Magazine to be useful in your research: