The socialists of the whole world solemnly declared in 1912 in Basel that they regarded the impending European war as a “criminal” and most reactionary deed of all governments, which must accelerate the collapse of capitalism, inevitably engendering revolution against it. War came, crisis came. Instead of revolutionary tactics, the majority of Social-Democratic parties followed a reactionary course, taking the side of their governments and their bourgeoisie. This betrayal of socialism signifies the collapse of the Second (1889–1914) International, and we must give ourselves an account of what caused this collapse, what engendered social-chauvinism, what gave it its strength.