Short answers, pulled from the story.
Otto von Bismarck implemented the world's first modern welfare state in 1883. This strategy known as State Socialism was a direct response to the Anti-Socialist Laws that had banned socialist organizations. The conservative Chancellor co-opted social reform ideas to reduce support for the Social Democratic Party of Germany.
The Russian Revolution of 1917 created an unbridgeable divide within the socialist movement that split the family into two distinct branches. Those who supported the October Revolution renamed themselves Communists while those who opposed the Bolsheviks retained the Social Democrat label. This split represented a fundamental disagreement on the means to achieve a communist society.
Eduard Bernstein a German social democratic politician fundamentally altered the trajectory of the movement by rejecting orthodox Marxism. In the late 19th century Bernstein argued that socialism should be grounded in ethical and moral arguments and achieved through gradual legislative reform. His ideas known as Marxist revisionism became mainstream after the First World War.
The Nordic model emerged as the defining feature of the golden age of social democracy with countries like Sweden Norway and Denmark implementing social corporatism. The Nordic countries including Denmark Finland Iceland Norway and Sweden rank highest on metrics of real GDP per capita and economic equality. This model is characterized by a corporatist system of collective bargaining and universal public services.
The lack of a strong and influential socialist movement in the United States has been linked to the Red Scare and the legacy of slavery. Racial stratification within the American working class produced a segmented labor market with distinct and often conflicting political priorities. This structural fragmentation undermined the development of class solidarity and posed a persistent obstacle to the formation of robust left-wing politics.