The 51st Sikhs (Frontier Force) was an infantry regiment of the British Indian Army. It was raised in 1846 as the 1st Regiment of Infantry The Frontier Brigade. It was designated as the 51st Sikhs (Frontier Force) in 1903 and became 1st Battalion (Prince of Wales's Own Sikhs) 12th Frontier Force Regiment in 1922. In 1947, it was allocated to the Pakistan Army, where it continues to exist as 3 Battalion The Frontier Force Regiment.

The Indian Army during British rule, also referred to as the British Indian Army, was the main military force of the British Indian Empire until 1947. It was responsible for the defence of both British India and the princely states, which could also have their own armies. As quoted in the Imperial Gazetteer of India, "The British Government has undertaken to protect the dominions of the Native princes from invasion and even from rebellion within: its army is organized for the defence not merely of British India, but of all possessions under the suzerainty of the King-Emperor." The Indian Army was an important part of the forces of the British Empire, in India and abroad, particularly during the First World War and the Second World War.