Spanish Civil War
The Spanish Civil War was a civil war in Spain fought from 1936 to 1939 between the Communists and the Fascists. It was also the first hot war in The World's War Against Communism. The "Republicans" were loyal to the Communist Popular Front government of the "Second Spanish Republic", and consisted of various, communist, separatist, anarchist, and "republican" parties, some of which had opposed the government in the pre-war period. The opposing Fascists were an alliance of Falangists, Fascists, monarchists, actual conservatives, and traditionalists led by Spanish patriots - among whom General Francisco Franco quickly achieved a preponderant role. Although there was street-fighting throughout the world, it was the first hot war in the battle between fascism and communism, and thus the beginning of WWAC.
Leading up to the war
After years of political polarization and tension buildup, a faltered military coup and various waves of political violence led to the true start of the civil war.
Foreign involvement in the conflict
Throughout the course of the conflict, many nations chipped in to support both sides of the Civil War, despite pledges of non-intervention all across Europe. This support included foreign legions, international volunteers, arms and munitions as well as direct financial aid.
Fascist
Italy and Germany
Italy and Germany were possibly the largest outside influences to the course of the war, sending up to date arms, munitions and planes along with well equipped forces such as the famed Condor Legion, Aviazione Legionaria (Legionary Airforce) and the Corpo Truppe Volontarie. Germany's support, while great, was limited due to fears of escalation as a result of blatantly violating the non-intervention treaty; Adolf Hitler encouraged Mussolini to contribute greater Fascist forces to compensate - Italy happily obliged to secure the position of Fascism in the Mediterranean, sending 50,000 men at their peak.
Others
Portugal was the runner-up in supporting the Fascist side, with between 8,000 and 12,000 volunteers making it to Spain and a number of weapons shipments being received through Portuguese ports. The involvement of Portugal would've been greater, were it not for political worries culminating in a communistic naval revolt of 2 ships. 1,052 Yugoslavs of various ethnicities volunteered, 700 Irishmen and a minute force of seven Romanian Legionaries also joined the fight against communism.
Republican
Involvement of the USSR
Bolshevik intervention in the conflict was, as is notorious with the Soviet Union, limited and sub-par. Unlike the Italian and German comrades of the Fascists, Stalin wished to keep his activities more covert. Material support came in the form of weapons smuggling -done through ships flying incorrect flags or displaying improper markings-, the dispatching of military advisers, a token fighting force of under 500 men and a meek attempt at organizing international volunteering efforts for the Republican side. Weapons delivered were said to be of varying quality, with the only reliably up-to-date technology provided being aircraft. Estimated material support includes between 634-806 aircraft, 331-362 tanks and 1,034-1,895 artillery pieces.
All of this was paid for with 510 tonnes of gold from taken from the Bank of Spain, around 72.6% of the bank's gold reserves. The ensuing monetary crisis was blamed on the free market by the Republicans. Fascists theorized that this economic strife was planned for in draining the nation's gold reserves, which could very well be true given the Republicans exchanged the remaining 156 tonnes for currency with France. If this was indeed a form of economic warfare, it backfired horribly on the Republicans, as at the height of the crisis prices for goods were up a stunning 1,500% in Republican controlled areas, and only 40% in Fascist controlled areas.
Poland
Poland was the fourth largest contributor to either side in the war behind the Soviet Union, Italy and Germany. Their involvement was almost entirely for economic reasons.
Mexico
Mexico refused to sign any treaties regarding the war, and sent a great deal of funds towards Republican Spain in the form of material and financial aid, around 2,000,000$ worth.
Anti-Christian atrocities
After the coup of July, 1936, nearly all Republican controlled areas experienced massive bouts of violent acts from sections from all leftist groups under the Republican banner. These waves of violence, known as the Terror Rojo (Red Terror) in Spain, resulted in 10s of thousands of deaths (Estimates range a massive 38,000 - 73,000 killed), nearly 7 thousand of those killed were priests.