How did Chester A. Arthur become president?

Chester A. Arthur became the 21st president of the United States after the death of James A. Garfield. Arthur was serving as vice president when Garfield, the president, was assassinated by a disgruntled job-seeker. He died two months later from infections, after which Arthur was sworn in as president.
Although Chester A. Arthur was a member of the Republican Party, he rose above politics. He signed into law the Pendleton Civil Service Act in January 1883, which mandated that government jobs were earned by merit, not by political connection, and that employees could not be fired for political reasons. Toward the end of his term, Garfield’s health began to suffer, which is why he did not seek re-election in the 1884 campaign.