Canada became a federation within the British Empire in 1867.
Quote from
http://en.wikipedia.org/wiki/History_of_Canada"On July 1, 1867, with the coming into force of the British North America Act (enacted by the British Parliament), the Province of Canada, New Brunswick, and Nova Scotia became a federated kingdom in its own right.[36][37] The term dominion was chosen to indicate Canada's status as a self-governing colony of the British Empire, the first time it was used in reference to a country."
I have quite a few pics of the Wellington Barracks which I found online & there is a marvellous giude book online for the Citadel.
http://books.google.com/books?id=IyZrV2eLT3wC&dq=halifax+citadel&printsec=frontcover&source=bl&ots=0ZZ3Dj3q20&sig=kGYwxj-qosPij2fCCXxbMUoKVQA&hl=en&ei=6AmcSs3TKZzSjAeiysHbDQ&sa=X&oi=book_result&ct=result&resnum=7#v=onepage&q=&f=falseSorry can't find the 'shrink link' feature!!
If Thomas joined the British Army his discharge papers should be at the Public Records office at Kew in the WO 97 series. However, you really need to know the regiment first and roughly when he was discharged. Presumably he would have joined about 1870 onwards? Which censuses or certificates do you have him as a soldier? See
http://www.nationalarchives.gov.uk/militaryhistory/army/?WT.lp=mh-33483and
http://www.nationalarchives.gov.uk/militaryhistory/army/step4.htmfor information.
Regiments at that time [or certainly in the 1850s to 1880s] were posted in a circuit - including Malta, GIbraltar, Halifax & Bermuda. If he joined a British regiment on posting to in Halifax that regiment may have been returned to Britain. Do any of his children's birth certificates [or dates of birth even] give the regiment? I can look on findmypast for 'Brook[e]s' children's births around 1875 - 1885 if you like.
If it was not the British Army I think the other force he could have joined was the Canadian Millitia [but I may well be wrong] see
http://en.wikipedia.org/wiki/History_of_the_Canadian_Armybut I have no experience of this. You could post on the Armed Forces board; there is also a marvellous yahoo group 'britregiments' full of people as knowledgeable & helpful as rootschatters [queries about regiments/army matters/army people only - no direct genealogy queries].
PM me with your email address & I will send you the pics.
Regards, Jean