The major event that led to the start of the French Revolutionary Wars was the Declaration of War against Austria on April 20, 1792. Tensions had been mounting between revolutionary France and monarchies across Europe, particularly Austria and Prussia, who feared the spread of revolutionary ideas. The French Revolution's radical changes in governance and social structure alarmed neighboring countries, leading them to seek the restoration of the French monarchy. The declaration marked the transition from internal revolution to external conflict, setting off a series of wars that would last for over two decades, reshaping Europe’s political landscape.