The history of the use of carrier pigeons in warfare is indeed a varied and interesting one, with a long and illustrious history. It is believed that the use of carrier pigeons as a messenger service had it origins in antiquity – over three thousand years ago by the Egyptians, Persians and Romans; in 1150 A.D., the Sultan of Baghdad strapped capsules filled with papyrus sheets to the leg or back feathers of pigeons, and used them as messengers. They were used as recently as 1990, by the Iraqi Army during the First Gulf War.
WWI

In terms of books written on the training and use of carrier pigeons for war, the commander of the Carrier Pigeon Service (CPS) in the British Army during WWI wrote a book about it. The book itself has excellent photographs and illustrations but the text itself is quite biased, sentimental and nostalgic, but this is to be expected of the time and of the author. It is Osman, Lt. Col. A.H., Pigeons in the Great War: A Complete History of the Carrier Pigeon Service during the Great War, 1914 to 1918 (London, 1928).

In this book, Osman describes his experiences in setting up the CPS in the British Army during the war. Carrier-pigeons had had a strong, albeit unofficial, tradition in the British military: carrier-pigeons were used to deliver the news of Wellington’s victory at Waterloo before the official (horse-mounted) couriers. However, in 1908 the British Admiralty decided to cease using carrier-pigeons and replace them with wireless sets. However, this was an optimistic rather than realistic endeavour and at the outbreak of WWI, the practical uses of carrier-pigeons became apparent.

Unlike the French or German Armies, the British Army did not have an official carrier-pigeon service prior to the outbreak of WWI. In September 1914, the French Army gave 15 pigeons to the British Intelligence Service, but these were not used on the battlefield. A carrier-pigeon service was initially improvised by the II Corps during the Second Battle of Ypres in May 1915. Shortly afterwards, the Carrier-Pigeon Service (CPS) was officially founded under the control of the Intelligence Corps and under the directorship of Captain (later Lieutenant Colonel) A.H. Osman. The CPS was transferred to the Director of Army Signals in June 1915 and thereafter was part of the Signal Service. The CPS was part of the Army but eventually the RNVR and RAF also acquired limited carrier-pigeon services. The Carrier-Pigeon Service was only used when telegraph and telephone communications failed and was soon overtaken by the development of Wireless Telegraphy (i.e. Radio), further limiting their usage; hence, they were only used for emergency or espionage purposes.

While carrier-pigeons did have particular successes with the British at the Battle of the Somme, and with the French at the Battle of Verdun, their main area of success was with the Intelligence Service.

Pigeons were pretty successfully used by the French at Verdun in the first days of the battle. In some instances they delivered messages of crucial importance. The distance they had to fly was comparatively short as they only had to reach the citadel at Verdun and were send out from one of the various Forts or Batteries.

The last Pigeon coming out of Ft. de Vaux was awarded the highest order of France for getting through with the vital message written by the commandant that the Fort was about to fall. The Pigeon made it despite the fact that it had to flow through fogs of poisoned gas and air turbulences caused by heavy shelling from guns up to the caliber of 42 cm (which could bring down an airplane).

The Pigeon died immediately after bringing the message to HQ. It was stuffed and is now in a French museum decorated with its order. A picture and monument can be seen at the remands of Ft. de Vaux at the preserved battlefield of Verdun.

Initially the British Armys CPS was attached to the Royal Engineers but was later transferred to the Intelligence Corps. Despite the fact that the CPS was only called into service when field telegraph or telephone systems had failed, there were over 22,000 pigeons, 150 mobile lofts and 400 personnel serving in the CPS at the end of the war. Over 100,000 pigeons in total had served Britain during WWI but despite this, the Royal Navy, Royal Air Force, and Army all demobbed their pigeon services promptly at the end of the war.

The reason for the British service being demobbed immediately after the end of WWI is probably because a regular force would not be able to maintain the necessary skills and knowledge, except at ruinous cost, compared to civilian enthusiasts who will maintain those skills without cost to the government.

It can be observed that the statistics for the CPS are slightly misleading: messages were often sent by two pigeons as well as more ordinary methods of communication. Despite this, it seems probable that the sizeable effort and resources expended on the CPS during WWI was justified due to certain conditions unique to the Western Front, and the fact that better alternatives were, as yet, unavailable.
WWII

At Corregidor in WWII a pigeon loft was installed and a private assigned to look after them but it were destroyed by artillery before they could be used.

The Australian Army made extensive use of pigeons in WWII in New Guinea and the islands.

Rugged terrain and unusual atmospherics meant that wireless communications often failed. Wireless communications could be intercepted, whereas it was virtually impossible to intercept a pigeon. A pigeon can carry a map, sketch or chart, which wireless cannot. And the birds very rarely failed to get through.

Bert Cornish persuaded the Australian Military Forces to raise a pigeon corps soon after the outbreak of WWII. The High Command was sceptical, perhaps failing to consider how often technology like wireless fails under operational conditions. Cornish strongly opposed moves to transfer recruits from general training, insisting on a small group those who had been civilian enthusiasts pre-war. 13,500 birds were donated by owners across Australia. Cross-breeding was carried on during the war, to get birds better able to withstand the climate of New Guinea and the islands.

The birds were soon in high demand by frontline troops. Also, Army supply boats insisted on carrying them.

Australian birds won two Dickin medals in WWII: The first flew 40 miles to Madang in a thunderstorm, after its boat foundered. The second covered 30 miles (partly through heavy fire) to bring news of a US company trapped by Japanese on Manus Island.

Owing to quarantine regulations, all the Australian birds were destroyed at the end of WWII.

I know of at least one British bird, Commando who won a Dickin Medal for carrying out three missions with the SOE in France. Apparently these birds often failed to get through, due to strong German counter-actions.

THE BERLIN AIRLIFT, 1948-1949.

A 1948 U.S. Army Signal Corps document discussing communications between the American sector in the isolated city and the U.S. Occupation Zone. The Department of Defense was seriously concerned that the Soviet Union might cut off all land lines between Berlin and the Zone and jam all radio communications, moves that would have made the Berlin Airlift exceedingly difficult if not impossible. The Corps concluded that carrier pigeons offered an alternative means of communication, should the Soviets take those actions.