Hari Singh Nalwa was Commander-in-chief of the Sikh Khalsa Fauj, the army of the Sikh Empire. He is known for his role in the conquests of Kasur, Sialkot, Attock, Multan, Kashmir, Peshawar and Jamrud. Hari Singh Nalwa was responsible for expanding the frontier of Sikh Empire to beyond the Indus River right up to the mouth of the Khyber Pass. At the time of his death, the western boundary of the Empire was Jamrud.

The Sikh Khalsa Army, also known as Khalsaji or simply Sikh Army, was the military force of the Sikh Empire. With its roots in the Khalsa founded by Guru Gobind Singh, the army was later modernised on Franco-British principles by Maharaja Ranjit Singh. It was divided in three wings: the Fauj-i-Khas (elites), Fauj-i-Ain and Fauj-i-Be Qawaid (irregulars). Due to the lifelong efforts of the Maharaja and his European officers, it gradually became a prominent fighting force of Asia. Ranjit Singh changed and improved the training and organisation of his army. He reorganized responsibility and set performance standards in logistical efficiency in troop deployment, manoeuvre, and marksmanship. He reformed the staffing to emphasize steady fire over cavalry and guerrilla warfare, improved the equipment and methods of war. The military system of Ranjit Singh combined the best of both old and new ideas. He strengthened the infantry and the artillery. He paid the members of the standing army from treasury, instead of the Mughal method of paying an army with local feudal levies.