Improvised incendiary devices were used for the first time in the Spanish Civil War between July 1936 and April 1939, before they became known as "Molotov cocktails". In 1936, General Francisco Franco ordered Spanish Nationalist forces to use the weapon against Soviet T-26 tanks supporting the Spanish Republicans in a failed assault on the Nationalist stronghold of Seseña, near Toledo, 40 km (25 mi) south of Madrid. After that, both sides used simple petrol bombs or petrol-soaked blankets with some success. -- Wikipedia.
Finnish soldier armed with a Molotov Cocktail.
But even with the knowledge that war might be coming, Finland could only muster a fifth of the manpower and a fraction of the modern weaponry of the USSR and had little advanced weaponry to defeat Soviet tanks. When the Soviets crossed the border in late November, their military planners anticipated a war would last for only a week or two. It lasted until March in no small part because of the Molotov cocktail.
The Molotov cocktail is an elegantly simple weapon, needing only a bottle and some flammable materials like gasoline or tar. Using either a basic fuse or even an old rag, a soldier sets the end on fire and hurls the weapon so the force of impact will shatter the glass and ignite the flammable materials inside. While the weapon was first employed during the Spanish Civil War in 1936, the Winter War was where it first became truly famous. It is also when the weapon got its modern name. The Finns sarcastically named it for the Soviet Foreign Minister, Vyacheslev Molotov, after he claimed Soviet bombers were dropping food aid to starving children in Helsinki. The Molotov cocktail was the perfect accompaniment to “Molotov’s bread baskets.”