Also known as the 100 Hours War, it was the 1969 war between El Salvador and Honduras.
Named so for the rioting that took place during the second round of the North American Qualifier for the 1970 FIFA World Cup.
It was fought over Honduran land reform and El Salvador's immigration problem, as well as a border dispute. 4,000 people died in the 4 days of fighting, and the war ended due to intervention from the OAS (no, not the OSS).
They signed a peace treaty 11 years after the war ended and Honduras won the disputed territory in a ICJ (International Court of Justice) ruling in 1992.
The ramifications of the war (the forced return of hundreds of thousands of salvadorans to a military-controlled El Salvador) would eventually become one of the reasons that led to the start of the eleven-years long (1980 - 1992) Salvadoran Civil War.
In the case of soccer, El Salvador won a spot for the World Cup, but lost in the Group Stages