To the chiefs of the protected Sutlej states issued on 13 December 1845, two days after the Sikh army had crossed the Sutlej, is a declaration of war against the State of Lahore. Fear of possible repercussions in the Sutlej Sikh states was responsible for this proclamation. The Proclamation accuses the Sikh army of having invaded the British territories. The British government, it maintains, had observed faithfully the conditions of the treaty of amity signed in 1809 with Maharaja Ranjit Singh since whose death the disorganized state of the Lahore government had compelled it to adopt precautionary measures for the protection of the British frontier. Active military preparations at Lahore had necessitated the advance of the British troops towards the Sutlej to reinforce the frontier posts. It was the intention of the British government to protect British territories, and punish the violators of treaties and the disturbers of public peace. The Proclamation also declared the possessions of Maharaja Duleep Singh on the left bank of the Sutlej confiscated and annexed to the British territories. The proclamation called upon the chiefs and the sardars in the protected territories to cooperate with the British government for the punishment of the common enemy, assuring them that the protecting power would promote their interests. Subjects of the British government, who shall continue in the service of the Lahore State, shall be liable to have their property on this side of the Sutlej confiscated, and have themselves declared to be aliens and the enemies of the British government.
The myth that the Sikhs had violated the treaty and broken the peace without provocation must be dispelled. Hectic military preparations on the part of the British across the Sutlej had led the Sikhs to forestall them. A British army 45,000 strong, with 98 guns, had been assembled at the Sutlej frontier; a warlike flotilla of 60 gunboats of 3 tons each and 56 pontoons to bridge the Sutlej for an invasion of the Punjab was ready at Firozpur. Movements of British troops could scarcely be concealed from the Khalsa army panchayats, who insisted on the surrender of Raja Suchet Singh's treasure buried in Firozpur, restoration of the village of Maurati, and a free passage for troops into their Sutlej possession which had been virtually seized by the British. Refusing to deal with the Khalsa, the British declared that there was no recognized head of the State. Already on 6 December, the British forces at Ambala and Meerut had moved forward towards Firozpur and Ludhiana; on 11 December they were still advancing towards the frontier when the Sikh army, in large numbers, crossed the Sutlej
Source:Encyclopaedia of Sikhism - Harbans Singh