The NHL is by far the best attended and most popular ice hockey league in the world, and is among the major professional sports leagues in the United States and Canada. The league's history began after Canada's National Hockey Association decided to disband in 1917; the result was the creation of the National Hockey League with four teams. The league expanded to the United States beginning in 1924 and had as many as 10 teams before contracting to six teams by 1942–43. In 1967, the NHL doubled in size to 12 teams, undertaking one of the greatest expansions in professional sports history. A few years later, in 1972, a new 12-team league, the World Hockey Association (WHA) was formed and due to its ensuing rivalry with the NHL, it caused an escalation in players salaries. In 1979, the 17-team NHL merged with the WHA creating a 21-team league. By 2017, the NHL had expanded to 31 teams, and after a realignment in 2013, these teams were divided into two conferences and four divisions. The league is expected to expand to 32 teams by 2021.