The following excerpt is from the May 1, 1938 newsletter called "Volunteer for Liberty", published by the Veterans of the Abraham Lincoln Brigade..  It gives a sense of who these people were and what those who survived believed in about the war against Franco.  The back issues of this news letter make fascinating reading and can be found here.
 
 
 

(...)
The Spanish people voted for democracy; Franco revolted against this
democracy. The Spanish people voted for schools, and land, and living
wages, The reactionaries of Spain rose In blood and bullets against these
demands. The Spanish people voted for peace and independence. They
have been answered by murder and a brutal attempt at foreign
domination.

Spain is a lesson no less for America than for all free and all threatened
peoples. Courage and self sacrifice we can learn from the Spanish
people, but we must join to this, organization and vigilance, that a like
revolt may not be made in this country. For this lesson, for the bulwark
the Spanish people are providing against the sweeping darkness of
fascism, for their devoted service toward saving America and all
countries from a world-wide slaughter, for these reasons the American
people owe the Spanish people their most generous and devoted support.
On May Day last year, the Veterans of the Abraham Lincoln Brigade
were in Spain, The day there was not for parades. Short meetings there
were, but in the trenches, in the training bases and hospitals, and
throughout the transport service the day was given to our regular tasks,
pursued with a heightened realization that the eyes of Labor the world
over were, especially on this day, upon us.

May Day this year finds 3,200 Americans still carrying on In Spain,
with the same devotion and the same courage. The thoughts of an
awakened America must be with them today; the assistance of an
awakened America must be extended to them.

It is fitting that we, the Veterans of the Abraham Lincoln Brigade,
should lead the May Day Parade and we are proud to do so. We have
taken part with the Spanish people in their determined struggle for those
objectives for which Labor all over the world is mobilized today. It is
right that we should, by our presence, remind New York and America of
what we have seen and of what we have fought for. It is right that we
should devote ourselves to the great task of determining that these things
shall not take place here.

We have seen with our own eyes the naked horror of fascism. America
may be assured that we shall not soon forget.

David McKelvy White