Because of this, players from the National Hockey League and its senior minor-league teams were not allowed to compete, while the Soviet Union was allowed to use permanent full-time players who were positioned as regular workers of an aircraft industry or tractor industry employer that sponsored what would be presented as an after-hours amateur social sports society team for their workers. In 1970, after an agreement to allow just a small number of its professionals to participate was rescinded by the IIHF, Canada withdrew from the tournament.[1] Starting in 1977, professional athletes were allowed to compete in the tournament and Canada re-entered. [2] The IIHF requires that players are citizens of the country they represent and allow players to switch national teams provided that they play in their new nation for a certain period of time.