The "Red Scare" following the First World War and the Bolshevik Revolution was used by elements in the Justice Department (in particular a young J. Edgar Hoover) to bolster their careers.  The two principals in the case were Italian immigrants and had been opposed to the war, and in fact fled to Mexico to avoid the draft.  The facts that they were immigrants, draft-dodgers, and attended meetings of supposedly "Red" political groups were all strikes against them in the minds of the average American citizen of the period.

The two did not have what could be termed perfect command of the English language, and obviously suffered confusion during the trial because of that.  The evidence for and against the two is itemized in one of the links below.  The fact that the witnesses for their alibis were all Italian immigrants weighed heavily against the accused, another example of the post-war isolationism of America.  Since many were also involved in "anarchist" politics, this also played into the hands of the prosecutors.  The entire episode is a rather sad example of American justice at one of its low points.  An ironic note is that although the trial and surrounding publicity was very biased, the truth is that one of the men was guilty, and the other apparently involved to some degree.

In 1977 Massachusetts Governor Michael Dukakis issued a proclamation absolving the two men, who had been executed 50 years earlier.  Although a popular move, this may not have been wise, since evidence subsequent to their executions indicates that while Vanzetti was innocent Nicola Sacco was guilty.  In 1943 Carlo Tresca, the anarchist leader originally chosen as their lawyer, claimed that to be the case.  Others involved in their defense also stated that Vanzetti was innocent of "actual participation" in the crime, but may have been involved in some way, but that Sacco was guilty in all respects.