Honduras won the first game (in Honduras). Then El Salvador won the second game (in El Salvador). When El Salvador won the third game in Mexico, all hell broke loose. Literally.
El Salvador was and is one of the most densely populated countries in the Americas. Honduras, in comparison, was and is sparsely populated. By the end of the 1960s, over 300,000 Salvadorians were living and working (often illegally) in Honduras.
The dilemma posed by these immigrants, many of whom cultivated previously unproductive land, was addressed through a series of bilateral agreements between the two Central American nations. The last of these agreements, conveniently, expired in 1969.
To make matters worse, the government in Tegucigalpa, the capital of Honduras, initiated land reform that effectively kicked Salvadorians off the land. Thousands fled back to El Salvador.
Then, El Salvador started claiming the land that had previously been held by its citizens in Honduras as El Salvador's. It was in this climate that the two countries met on the soccer field to determine who would qualify for the 1970 World Cup in Mexico.