After 1947 the Akali Dal demanded a state within India whose official language would be Punjabi in Gurmukhi. The States Reorganisation Commission of 1955 rejected it, on the ground that Punjabi and Hindi were not sufficiently distinct and that the demand was religious rather than linguistic.
The census of 1951 and 1961 recorded large numbers of Punjabi-speaking Hindus in the province returning Hindi as their mother tongue, after a campaign by Hindu organisations urging them to do so. That is the single fact that made the linguistic case impossible to argue cleanly, and it is the fact most often left out.
Morchas were run through the 1950s and 1960s with tens of thousands of arrests. Sant Fateh Singh and Master Tara Singh both undertook fasts. The Akali Dal split over method more than once.
The Punjab Reorganisation Act of 1966 divided the state three ways. Haryana was created, the hill districts went to Himachal Pradesh, and Punjab was left with about two fifths of its 1956 area. Chandigarh, built as the new capital after Lahore was lost, was made a union territory shared as capital by both states.
The river waters and Chandigarh were left unresolved in 1966 and are unresolved now. Every Punjab agitation since has had those two items in it.
The Sachar formula of 1949 split the state into a Punjabi speaking zone and a Hindi speaking zone for school instruction and satisfied nobody. The Regional Formula of 1956 tried again with two regional committees inside one assembly. Both were arrangements for not answering the question.
Master Tara Singh fasted in 1961 and broke it without the demand being met, which cost him the leadership. Sant Fateh Singh fasted in 1965 and the announcement came the year after. What changed between those two dates was a war with Pakistan fought largely on Punjab's ground and with Punjab's soldiers.
The Act of 1966 left the Bhakra Beas works and the headworks outside the new state's control, gave the capital to neither state, and set up separate machinery for allocating the rivers. Every one of those is a live dispute now, and every one of them was a deferral rather than a decision.