This is a list I compiled of the veterans on the Titanic from wars before the disaster and for wars afterwards. The Titanic is seen as marking the end of the Edwardian Era and beginning the pre-WWI era leading up to when the first shots were fired, changing the geopolitical landscape of the world forever. With WWI happening a few years after the disaster, many survivors got caught up in it. The list may not have a few names, which can be added as I encounter more veterans or future veterans on board the Titanic. But this is the most complete list I've encountered thus far.
American Civil War
Union:
1st class passenger Francis Davis Millet was a drummer boy and surgical assistant for the 60th Massachusetts Calvary
1st class passenger George Goldschmidt served in Harper's Ferry and Gettysburg in the 22nd New York National Guard.
1st class passenger Edward Gifford Crosby served in the 1st Michigan Cavalry in multiple battles including Second Bull Run, Yellow Tavern, Wilderness, and Gettysburg. He was joined on September 14, 1861 and was mustered out on September 4, 1864. He re-enlisted and became a part of Hancock’s Veterans Corps until he was mustered out on April 14, 1866.
Confederacy:
1st class passenger Isidor Straus was a blockade runner
Notable mention:
1st class passenger Archibald Gracie IV's father, Archibald Gracie III, was a brigadier general who gained some fame for his brave actions during the war including charging at the Union during the Battle of Chickamauga up a hill and continuing to fight them after considerable losses. He was killed when Gracie IV was a small boy during the Civil War and Gracie IV spent his life honoring his father's legacy. This culminated in him writing a book called "The Truth About Chickamauga" and he was on the Titanic following that. He talked to Isidor Straus about the Civil War while they were on the Titanic.
2nd Boer War
British Army:
1st class Saloon Steward Steward Frederick Dent Ray Private-Cape Mounted Police
2nd class Saloon Steward William Edwy Ryerson Private-Royal Canadian Dragoons
3rd class Steward John Edward Hart South African Light Horse
3rd class Steward Arthur Read Lewis
Private Storekeeper Charles Morgan Quartermaster-Western Province Mounted Rifles
3rd Butcher Frank John Roberts Trooper
Band member Pianist William Theodore Brailey
Junior Assistant Engineer Herbert Gifford Harvey Private-46th Company Imperial Yeomanry
Fireman William Clark Private-Brabant Horse
Fireman James Carter Farrier-Sergeant of the South African Mounted Police
Fireman Charles William Lindsay Lance Corporal
Royal Marines:
Trimmer Samuel Collins Private
Mercantile Marines:
Captain Edward John Smith was the Captain of the Majestic for 2 trips transporting troops to South Africa. He was awarded the Transport Medal and South Africa Clasp
Chief Purser Hugh Walter McElroy Served on board the troopship Britannic. He was awarded the Transport Medal and the South Africa Clasp.
American-Philippine War
Americans:
1st class passenger Archibald Butt was a Quartermaster in the Philippines in which he met Governor Taft.
Spanish-American War
US Army:
1st class passenger John Jacob Astor IV
1st class passenger Col. John Weir
1st class passenger Philipp Mock
US Navy:
1st class passenger Erik Lindeberg-Lind
WWI
Royal Navy:
4th Officer Joseph Boxhall
5th Officer Harold G. Lowe
Quartermaster George Rowe
ABS Edward John Buley served as ABS 213566 and was on board the Victory, Hecla, and HMS Partridge on which he was KIA.
ABS George Moore
Lamp Trimmer Samuel Hemming served on the Victory, Roedeau, and Whitby Abbey.
Firemans' Messman Thomas Knowles
David Blair (former 2nd officer of the Titanic) served on the Oceanic and was court-martialed for a navigational error leading to her being grounded.
RN Reserve:
2nd Officer Charles Herbert Lightoller served as a Lieutenant and Commander on board HMS Garry , RMS Oceanic, RMS Campania, HMTB 117, HMS Falcon.
Fireman James Crimmins Trimmer ST2621
Merchant Navy:
ABS Walter Thomas Brice
ABS Frederick Charles Clench
Seaman 2nd class Saloon Steward Charles Edward Andrews Plate
Steward Arthur Victor Edwards Burrage (He was also in the RNR)
Trimmer George Henry Cavell
Fireman Arthur John Priest served on the RMS Austrais, HMS Alcantera, HMHS Britannic, and the SS Donegal.
Lookout Archie Jewell served on the HMHS Britannic and afterwards, the SS Donegal as an ABS when it was acting as a troop ship. He was killed in the sinking after being torpedoed by a German submarine.
ABS William Chapman Peters
Fireman Frederick Doel
Bed Room Steward Alfred George Crawford
British Army:
Steward Sidney Daniels
1st class Saloon Steward Frederick Horace Crafter Trooper 4426-Guards Machine Gun Regiment. He was awarded the British War Medal and the Victory Medal.
Fireman John Diaper was in the Hampshire Regiment Private 6696 before being wounded and served the rest of the war as a cook. He was in the Devonshire Regiment (Private 19378), Royal Engineers (Private 325365), and Private WR303003.
Kitchen Porter Reginald Hardwick was trained to be in the Army Service Corps MT (Private M/350600), but died of illness before he was able to take active duty.
Victuallating Crewmember Harold Prior was in the Royal Marine Light Infantry
Fireman Christopher Shulver/John Dilley was a driver (L/13321) for the Royal Field Artillery 39th Divisional Ammunition Column, 14th, 5th Reserve Army Brigade. He was afterwards sent to France where he was wounded by a shell in the shoulder in October 1917.
Fireman William Clark(?) was reported by newspapers to have rejoined the British Army during WWI. There is no record of him after that.
1st class cross-channel passenger Father Francis Browne was a chaplain and photographer
3rd class passenger Gurshon Cohen was shot twice in the same day, leaving him blind in one eye.
Elen Walcroft was a forewoman cook for Queen Mary's Army Auxillery Corps.
UK Army Service Corps:
Quartermaster Robert Hichens
US Navy:
3rd class passenger William Henry Tornquist
US Army:
3rd class passenger Daniel Buckley KIA
1st class passenger Robert Norris Williams
1st class passenger Dickinson Bishop
1st class passeger Robert Daniel
1st class passenger William Greenfield
1st class passenger William Mellors
1st class passenger Jack Thayer
1st class passenger John Pillsbury Snyder (Major in the US Infantry)
3rd class passenger Einar Karlsson
3rd class passenger John Kennedy died of Anthrax while training.
3rd class passenger Thomas McCormack
3rd class passenger Bernard McCoy April 3, 1918-June 12, 1919.
Canadian Military:
Major Arthur Peuchen was in the Queen's Own Rifles
French Army:
Louis Gustave Joseph Lesueur
Belgian Army:
3rd class passenger Jules Saap
3rd class passenger Jean Scheerlinck
British Red Cross:
1st class passenger Edith Pears
Stewardess Violet Jessop served on board the HMHS Britannic
American Red Cross:
1st class passenger Helen Churchill Candee
WWII
Royal Navy:
Victuallating Crew Member Harold Prior
Merchant Navy:
Fireman James Crimmins served as night watchman and survived the sinking of the Windsor Castle.
US Navy:
3rd class passenger William Henry Tornquist
Veterans between wars:
2nd Officer Charles Hebert Lightoller was a sub Lieutenant in the Royal Navy Reserve in 1903.
William Baird Silvey
William Chapman Peters was in the Royal Navy and served on the Jupiter and HMS Hercules prior to the Titanic.