Short answers, pulled from the story.
The assassination of Lieutenant José Castillo by Falangist gunmen in Madrid triggered the immediate military uprising that began the Spanish Civil War. This political murder catalyzed a chain reaction where state police executed conservative politician José Calvo Sotelo, solidifying the justification for the rebellion.
Just over half of the approximately 15,301 officers rebelled against the Popular Front government while the rest remained loyal. Officers who had fought in North Africa between 1909 and 1923 known as africanistas tended to support the coup while those who remained in mainland Spain called peninsulares generally stayed loyal.
The Republican zone experienced a profound social revolution where workers and peasants implemented socialist organizational principles through collectives organized by trade unions. This included the socialization of industry and the concentration of personnel into bigger plants or coordinated cartels.
Germany and Italy officially signed the Non-Intervention Agreement but ignored the embargo to support the Nationalists with aircraft, tanks, and troops. The Soviet Union provided arms to the Republicans while Britain and France led a political alliance of 27 nations that pledged non-intervention.
Franco entered the capital and declared victory on the 1st of April 1939 after leading a military coup against the Republican government on the 5th of March 1939. The Nationalists won the war in early 1939 and ruled Spain until Franco's death in November 1975.